Not sure I understand your question right, but are you asking for the events that lead up to this moment? Lord Krishna (an avatar of Vishnu in his mortal form) gives a huge discourse to Prince Arjuna which is called the Bhagavat Gita. The Gita in itself is a smaller (tiny in relation) subset of the epic called the Mahabaratha. To understand what happens in the Gita, you would need a tl;dr'ish version of the Mahabarata in itself.

The Mahabaratha is an interesting epic in that it does not follow the standard structure you find in most other epics. Here is a source that gives you an idea about the standard structure.

They key to a lot of the early and indeed, even modern movies / epics is (from the link),

Epics As Moral Codes (Or Political Propaganda): The epic nearly always:

1. Represents moral ideals and taboos in the behavior of the hero and antagonist. The hero's behavior and the lessons he learns along the way represent the culture's ideals; what the hero does, all men should strive toward. The Other (monster or antagonist) is shown as essentially/inherently inferior to the hero; the Other represents either those who break moral taboos or the inferiority of Other cultures/peoples/nations etc.

The Mahabaratha tosses all this moral correctness out the window. The bad guys aren't bad guys in the classical sense and the good guys are well...as flawed as the bad guys.

Duroydhana the main bad guy of sorts was a very wise, just ruler and praised by his subjects. Karna his best friend (and on the bad guys camp) was arguably the purest character in the epic and had no flaw but...his extreme sense of loyalty (which would be a virtue in any other epic) was his biggest flaw.

Take the so called good guys,

Krishna (the main protagonist and the mortal avatar of Lord Vishnu), he was short tempered, he cheated, he lied and wasn't averse to breaking all moral codes and using his godly powers to kill opponents (he was stopped from doing it though). Yudhistra, the noble king who doesn't ever lie, well he is a gambler who staked his own kingdom, brothers, wife and lost them all. Wins them back, and stakes them all again promptly.

This is the context to the characters that populate this epic.

Moving on to the epic itself, it is a tale of the 100 Kaurava brothers (the possible antagonists) and their war of succession against the 5 Pandava brothers. To cut a long story shot, the Kaurava's (especially the eldest, Duryodhana) aided by his scheming uncle Shakuni (he also has a back story that puts his scheming ways in a different and more moral light) cheats the Pandava's of their rightful kingdom.

After a lot of travails (including an extended exile in the wilds of India), the Pandava's come back to stake their claim. Duryodhana refuses, and he is backed (either willingly or silently) by the entire Kuru (kingdom that is under dispute) court.

The Pandava's throw down the gauntlet, and armies are called on both sides. Vassals summoned, and two massive armies meet on the battlefield of Kurukshetra.

The best warrior in the Pandava side is the Prince, Prince Arjuna and his charioteer is the Lord himself in his human form - Lord Krishna. Terms are discussed, the rules for the war are framed (infantry will fight only infantry, cease fire at dusk, wounded will be tended to irrespective of their side etc etc). Yudhistra (he was despite his failings, very noble) tries one last attempt at peace and asks for 5 villages. Duryodhana wants it all and refuses and the battle is on.

Prince Arjuna had sought boons from many a god to add to his already formidable base martial values and was the pivot around which the Pandava army revolved....and he gets cold feet.

Picture this, the battle lines are formed. Pin drop silence just before the battle and this mightiest of mighty warriors throws down his bow and arrow, shrugs his armour and tells his King that he ain't down to fight. Fighting would mean killing his own cousins, his own teacher, his own uncle, his friends (who had joined the Kaurava side). Killing all these people would be 'ADHARMA" (against honour) and walks back to his lines.

That is when Krishna begins the sermon that is now called the Gita. In this he essentially (very summarized version, a tl;dr if you will) tells Arjuna that not doing his duty would be ADHARMA. That every person in this world has a duty, a job and to not fulfill those duties is ADHARMA. If Arjuna's duties involved massacring all his relatives, that is what it is. Krishna then goes onto talk about Karma (not the internet points, but real Karma), and the choices humans make and how illusionary it is. He says that the entire world is "Maya" (illusion) and pain, pride, hurt, love are all facets of this illusionary world.

Arjuna is still not convinced (the sermon segues into a dialogue between the two) and to prove his point, Krishna becomes the Immortal God Vishnu.

Arjuna goes to war and as was his duty, slaughters tens of thousands, including his own relatives & friends.

The Pandavas rule the kingdom and as promised by Krishna, he meets them all in heaven (they all ultimately achieved their purpose on earth as deemed by the god) and it ends happily ever after. It would be interesting to note that the most noblest man in the tale is the only one to actually see a glimpse of heaven.

To the Mods - Sorry, but there are no sources I can provide (aside from the Mahabaratha / Gita translations themselves) though this is a parent comment. This is my understanding of the epic and has been framed by reading multiple versions of the above mentioned epic.

To OP - you could please visit /r/India and post a question there and you will have a lively debate that might answer your questions or raise yet more questions.

You're using J. Robert Oppenheimer's famous translation of it. Oppenheimer (the "father of the atomic bomb") had an idiosyncratic translation and potentially an idiosyncratic interpretation. I've written at some length here about what Oppenheimer himself probably meant when he referenced that line.

The TL;DR version is that Oppenheimer was referring to the scene where Krishna unveiled his true, terrible form to Arjuna, the prince who he had been cajoling into joining a battle. Oppenheimer is Arjuna in his version of things, not Krishna. The bomb is Krishna.

To quote myself:

So let’s step back and ask who Oppenheimer is meant to be in this situation. Oppenheimer is not Krishna/Vishnu, not the terrible god, not the “destroyer of worlds” — he is Arjuna, the human prince! He is the one who didn’t really want to kill his brothers, his fellow people. But he has been enjoined to battle by something bigger than himself — physics, fission, the atomic bomb, World War II, what have you — and only at the moment when it truly reveals its nature, the Trinity test, does he fully see why he, a man who hates war, is compelled to battle. It is the bomb that is here for destruction. Oppenheimer is merely the man who is witnessing it.

Hijiya argues that Oppenheimer’s sense of Gita-inspired “duty” pervades his life and his government service. I’m not sure I am 100% convinced of that. It seems like a heavyweight philosophical solution to the relatively lightweight problem of a life of inconsistency. But it’s an interesting idea. It is perhaps a useful way to think about why Oppenheimer got involved with so many projects that he, at times, seemed ambivalent about. Though ambivalence seemed readily available in those days — nobody seems to be searching for deep scriptural/philosophical justifications for Kenneth Bainbridge’s less eloquent, but equally ambivalent post-Trinity quote: “Now we’re all sons-of-bitches.”

... I think the actual context of the quote within the Gita is far deeper, far more interesting, than the popular understanding of it. It isn’t a case of the “father” of the bomb declaring himself “death, the destroyer of worlds” in a fit of grandiosity or hubris. Rather, it is him being awed by what is being displayed in front of him, confronted with the spectacle of death itself unveiled in front of him, in the world’s most impressive memento mori, and realizing how little and inconsequential he is as a result. Compelled by something cosmic and terrifying, Oppenheimer then reconciles himself to his duty as a prince of physics, and that duty is war.

Oppenheimer's translation is a personal one, derived (one suspects) from Arthur Ryder, his Sanscrit teacher at Berkeley, who had his own idiosyncratic translation of the relevant passages:

A thousand simultaneous suns

Arising in the sky

Might equal that great radiance,

With that great glory vie.

Arjuna is awestruck and spellbound:

Amazement entered him; his hair

Rose up; he bowed his head;

He humbly lifted folded hands,

And worshipped God. . . .

And then, in his most amazing and terrible form, Krishna tells Arjuna what he, Krishna, is there to do:

Death am I, and my present task

Destruction.

All of the above is separate from what a modern, practicing Hindu, or Sanscrit scholar, might think about the passage (there are a variety of different interpretations).

fyi, you may find this earlier post of interest:

http://www.reddit.com/r/AskHistorians/comments/1ny2sl/what_is_the_significance_of_the_use_of_i_am/