I know that this sub does not allow speculation, but I'm having a tough time finding a peer-reviewed source that agrees with me, so please correct me if I'm wrong!

Might it be due to the pressures he faced from his people after all those millenia? He was a pacifist for dozens of centuries while his people were starving and unhappy in their crowded cities. Genghis Khan's and Alexander the Great's encroachments into his territory probably pushed him over the edge; his people wanted more land to move into, thus forcing him to build up his military might to be greater than theirs. Additionally, Bismarck and Sejong were quickly gaining technological advancements that he could not keep up with. Surely the pressures made him snap into an all-out retaliation. It's a classic case of a leader with good intentions becoming violent.

Sources:

Goethe, The World's Mostest Literate People

Ibn Battuta, The People With the Pointiest Sticks

Pythagoras, People Who Like to Smile the Most

His birth name was Mohandas Gandhi; the honorific title "Mahatma" means "Lover of Nukes".

Now, we can argue until the cows come home about whether the title was given to him because of his love of nukes, or whether it was a misinterpretation of a different title that stuck and thus triggered his research and love of nukes. There's certainly a theory that people with certain distinctive names are more likely to choose a career that's at least superficially related to their name; someone with the surname Hamburger may be more likely to become a chef, someone with the last name Nurse may be more likely to go into medicine, and someone with a title that means "Lover of Nukes" may become a hyper-aggressive warlord.

Gandhi tells us himself... Sort of. Regarding the Bhagavad Gita, he said: "When doubts haunt me, when disappointments stare me in the face, and I see not one ray of hope on the horizon, I turn to Bhagavad-gita and find a verse to comfort me."

The Bhagavad Gita is a dialogue between Arjuna (the leader of an army) and Krishna (his charioteer, also God in disguise). Arjuna asks Krishna if the imminent war with his family is the right thing to do. Krishna answers that it is (although there's slightly more nuance to it than that and it takes about 150 pages). Then Arjuna fights the Battle of Kurukshetra. Nearly all warriors on both sides die (around four million), with only a handful of main characters left standing. The battle itself is not technically part of the Gita, but it forms an important context.

So to sum up, in times of doubt, Gandhi turned to a text where a national leader asks God if it's alright to slaughter enemies by the millions, while losing millions of his own people in the process. Then he does so. The question isn't "why was he obsessed with nukes," it's "why are we still alive to ask?"

I'm afraid the answer to that goes beyond my expertise.

Edit: you asked about Gandhi's character, and whether it conflicted with his nuclear policy. While I can't give you a definitive answer, I can tell you that he was far from the only leader to display such a conflict. Take Isabella of Spain, a devout Roman Catholic who, throughout her life, confessed the Christian faith and advocated, in her writings and especially her biography, the forced Christianization of foreign peoples and the expulsion of the Jews and Muslims. Despite this, she made Judaism the state religion (with the justification "Both Christianities are taken and this [Judaism] has the prettiest symbol'") and even had her diplomats lobby to make it the world religion. People in her own court lobbied her to "load down Sons of Abraham and expel the Jews," but her only reply was the paradoxical statement "This is not yes what thou!" (The enigmatic "yes what thou" has come down to us verbatim as "Si que tu!"). "Si que tu" (or rather an acronym that phonetically sounds like "si que tu" when pronounced in English) has since become a rallying cry for advocates of ethnic cleansing, religious violence, political assassination, eugenics, and (strangely enough) sibling marriage.

So when you get down to it, Gandhi's legacy isn't that weird.

Gandhi the First, was the the most non aggressive leader of all time. Early in his career, no conditions existed that would cause him to ever act hostile toward his neighbors, even if they were colonizing near his cities. However, something very strange happened to Gandhi the First when he reformed his country and adopted Democracy. Gandhi became the most warlike, aggressive, warmonger-er to ever live. Gandhi declared war on every country in the world and fired ICBMs at every major city. Most people wrongly think that nukes corrupted Gandhi, but in fact he had spent many years as a King sitting on ICBMs for defensive purposed without anyone realizing until he became President.

Psychologists theorize that this has to do with the 255 Principle. Basically, due to the limitations of the human mind, there are only 255 states of aggression. We all know living in a Democracy will lower someone's aggression by about 10, on a scale of 1-255. Psychologists believe Gandhi was born with such low aggression, probably less than 10, that when he started living in Democracy, his aggression actually swung around, having no where to go, but to the 250ish range. Basically, Gandhi was so passive it turned out to be a serious personality flaw latter in life.

Gandhi the Second and Third, probably is better asked at r/theology. But it appears that God has a twisted sense of humor.

Ooh, finally one I'm qualified to answer!

So, let's recall Gandhi's other traits? What's he known for? He is best known for leading peaceful protests against British rule in India. Gandhi is known for loving peace. Unfortunately, because of warmongers, peace is a rare thing in this world. So, guess what Gandhi hates the most? Yep, that's right. Warmongers. And when people warmonger, he needs to punish them to preserve peace. As many IR theorists will tell you, nuclear weapons are the best deterrent. The fear of having a city obliterated by nukes will scare many leaders into peaceful behavior. The threat of mutually assured destruction has created peace, and reduced the incidence of warfare. However, some civilizations continue to warmonger, throwing caution to the wind. To make sure his threats are credible, Gandhi is forced to nuke these warmongers to restore peace.

Also, at the risk of breaking the rule against moralizing, Gandhi saw you nuke Japan, Capture Germany's capital, and wipe out the Iroquois entirely, so it's not like you didn't deserve it.

I can't source this other than some layman's research through the medium of historical simulation games, but seeing as there aren't any appropriate answers here, I'll give it a shot.

It has entirely to do with India's history under British colonial rule. The Indian subcontinent is (relatively) isolated from the rest of the world by land. The peaks of the Himalayas prevent travel for most land-based units, aside from a few exceptions like the Nepalese, Bhutanan, and Carthaginian civilizations. The eastern border of the subcontinent is made up of thick jungle, difficult to traverse for most units, and the Khemer civilization has always been rather weak. The North western frontier is more porous, as evidenced by repeated invasions, but India's unique "assimilation" ability toward civilizations with smaller populations meant that the effects of the influx of nomadic civilizations is relatively minor (the Arab camel archer invasion nonwithstanding).

However, India was hideously vulnerable to a naval invasion, and for some reason didn't heavily invest in naval technology. This let the British, Dutch, and Portuguese, whose Ships of the Line, Sea Beggars, and Carracks easily outclassed outdated Indian ships and captured coastal cities. Moreover, the tech bonuses of these civilizations made Indian assimilation bonuses negligible, and India remained a colony of the British (once they destroyed their rivals) for a considerable amount of time.

Although Ghandi won independence via nonviolent actions, India just doesn't have the resources to heavily invest in naval technology. Ghandi knows this, and the sea is still really the only frontier on which India is vulnerable. So Ghandi was so obsessed with nuclear weapons because it was the only way that India could threaten nations with naval superiority over India. And this doesn't even touch the issue of Pakistan...

He once stated: "militarism and nuclear weapons are the blood-soaked sign posts of a vanishing era." However, a country in a world that was filled with weaponry and militarism would never last. Thus, he believed in order to protect India from the world it may be necessary to be on the same playing field as them, militarily speaking. He wasn't obsessed with nuclear weapons specifically, he was obsessed with protecting the people of his nation.

Source: Wolpert, Stanley A., and Stanley Wolpert. Gandhi's passion: The life and legacy of Mahatma Gandhi. Oxford University Press, 2002.

Naess, Arne. "Gandhi and the nuclear age." (1965).

Well, in short form, ruling an empire for millennia can cause detrimental effects to a ruler's mind. While he started out in 1000 BC as a benevolent ruler who only wanted the best for his people, by 1945 he had become an insane megalomanic bent on world domination after democracy pushed him over the edge.

It is important to note that Ghandi himself probably never had his finger on the button. Ghandi's stated personal policies towards revolution and governance were indeed non-violent, but he also was quite clear that he would never stop other people from using violent means to achieve those ends.

Basically I'm willing to bet that Ghandi would just kind of circle an area on a map and go "Oh well, time to go peacefully starve myself in this backwards degenerate country next door in order to convince them that submission to my enlightened rule is the best option, SURE WOULD BE NICE IF I DIDN'T HAVE TO" and his subordinates would fall over each other hitting the launch button.

I can't back it up, but I believe that Gandhi at some point became so peaceful that he actually lapped the cycle of death, rebirth, goodness and evil, and became a war mongering monster.

He took Samsara too far man, he took it too far.

Gandhi was deeply affected by his disastrous invasion of backwards and isolated France, which ended when the Third Indian Army, also known as the "Stack of Doom", composed of many Tank divisions, was completely destroyed by an French force armed primarily with spears. This convinced the Mahatma that modern weapons were useless even against primitive peoples and that he was unable to protect his people.

Gandhi's behavior was increasingly erratic after India's adoption of democracy; it was around that time that the he began making cryptic remarks about "achieving ultimate peace" (Meier, 1989). The real meaning of those words was not revealed to the world until the day when the entirety of India's nuclear arsenal was launched simultaneously, in what was later known as "Gandhi's rage quit".

The events in Paris probably played a major part in his decision to usher a nuclear winter. Nonetheless, Gandhi's conclusions are disputed by many military analysts who argue that, however unlikely, Napoleon's victory in Paris was still possible, especially considering the his spearmen were somehow able to adapt their anti-cavalry tactics against modern armor, but that does not change modern weaponry general superiority. This is why the consensus among the (remaining) historians is that Gandhi belongs in the Hall of Shame of history, below even Dan Quayle.

Ahhh, but you see, it isn't actually at odds with his historical character. Gandhi was a proponent of non-violent resistance, so it seems to me that Nuclear Weapons would be the most effective non violent method of countering the oppression of foreign culture and/or trade routes. Backing his words with the threat of nuclear annihilation would cow other rulers to end their aggressive and imperialistic ambitions. It also probably, and reasonably appeared to Gandhi to be an effective way to eliminate threats to global peace. For example, if a warmongering ruler were to initiate violence by invading, resisting the reasonable demand of 3 cities, or allowing a open borders agreement to expire, then this warmongering menace to a peaceful world could be quickly neutralized without extended violence, as nuclear wastelands pose no significant threat to a harmonious coexistence with others.

There is this though: "Had we adopted non-violence as the weapon of the strong, because we realised that it was more effective than any other weapon, in fact the mightiest force in the world, we would have made use of its full potency and not have discarded it as soon as the fight against the British was over or we were in a position to wield conventional weapons. But as I have already said, we adopted it out of our helplessness. If we had the atom bomb, we would have used it against the British"

looking at the first few google links to find this "gandhi if we had the atom bomb british" - I can't find any contextualisation beyond it being a speech at a prayer meeting in 1947, so, is there very important context I'm missing here? (from here from /r/civ, but since I'm here~)