I am not going to get too much into this, but there are a few things in your question that I think need clarification.

he overthrew the Tsarist regime which was a feudalist regime

Lenin was not in Russia at the time of the February Revolution that overthrew the Tsar. Remember that 1917 saw two revolutions in Russia. The first came in 1917 and was already somewhat socialist in character. However, the Provisional Government that took over after that point made some decisions that made a good number of people angry at them, one of the most important being that they decided to honor Russia's previous alliances and continue the fight in World War I. The Bolsheviks, led by Lenin, promised to get Russia out of the war. They also took a more radically socialist position. They argued that the provisional government was too bourgeois (more or less) and that the true power should lie with the soviets (hence the famous cry "All Power To The Soviets!")

So, Lenin really did not have a lot to do with the overthrow of the Tsarist regime itself.

Lenin was honestly attempting to do what was best for the majority of people.

I think this is probably true, but the question you should consider is whether or not intention is the key factor here. Lots of people have had good intention and done fairly terrible things.

Towards the end of his life his health failed him, and Stalin succeeded him. I'm well on board for Stalin being a terrible person, however is it not true that Lenin was very critical of Stalin and Stalin's theories?

Lenin was critical of Stalin as a person, certainly. He did not think Stalin would be a good person to take over for him. The source that most people (including myself) would cite on this topic is Lenin's testament, where he wrote

Comrade Stalin, having become Secretary-General, has unlimited authority concentrated in his hands, and I am not sure whether he will always be capable of using that authority with sufficient caution

and then later

Stalin is too rude and this defect, although quite tolerable in our midst and in dealing among us Communists, becomes intolerable in a Secretary-General. That is why I suggest that the comrades think about a way of removing Stalin from that post and appointing another man in his stead who in all other respects differs from Comrade Stalin in having only one advantage, namely, that of being more tolerant, more loyal, more polite and more considerate to the comrades, less capricious, etc. This circumstance may appear to be a negligible detail. But I think that from the standpoint of safeguards against a split and from the standpoint of what I wrote above about the relationship between Stalin and Trotsky it is not a [minor] detail, but it is a detail which can assume decisive importance.

Lenin's criticism was, therefore, not chiefly ideological in nature. He was worried about him having too much power and was ultimately, it turns out, right about him abusing that authority. He is also concerned about the potential of a split in the party.

My history textbooks frame him as a war-monger and mass-murderer. Maybe less so the latter, but I can probably cite a few times he's called a war-monger in a few of my books, listed along-side people like Hitler, Mao and Stalin. How does a war-monger lead a country out of war? Didn't he aim for peace?

Well, Lenin certainly had absolutely no qualms about using violence to deal with people he felt were enemies of the revolution. Lots of enemies of the revolution were ordered executed and approved of by Lenin. Did he genuinely believe he was doing the right thing? I think probably. Lenin was not the only one who thought this was the right thing to do. But he certainly approved of violence as a way getting rid of enemies of the revolution. One of the big differences between Lenin and Stalin in practice here is that Stalin turned that violence back on the party in a very arbitrary way.

The Civil War was fairly brutal and enough people saw the Bolsheviks as a threat that it was not only a war about Bolshevism vs. Monarchy. There were monarchists, liberals, reactionary conservatives and even some non-Bolshevik socialists that formed what I would have to call a very tenuous anti-Bolshevik "alliance." Was Lenin a "War Monger"? I don't think so, but I have not really read that anywhere. Maybe it is in some textbook you have, but frankly textbooks are not very good historical sources to begin with.

It's also worth noting that the Bolsheviks did some very worthwhile things in the 1920s as well. They went a long way towards electrifying the country. They introduced mass education and launched a massive literacy campaign in the country side (which was not always well received I should add). Which leads to the next point. They industrialized the country's economy (at great human cost) which was ultimately very important.

But when you study the Soviet Union, you eventually start to get comfortable with dealing with contradictions, because so many abound in its history. Was Lenin quite brutal? Yes. Did his policies end up benefitting a lot of people? Yes. Did Stalinism come directly out of Leninism? Yes and No.

But the question that drives most of the debate in my experience is one that actually isn't a historical question at all. The question is, essentially "Wouldn't people have been better off with a capitalist/bourgeois revolution instead of a communist one." It all boils down to that really when this topic comes up in conversation. It's a big "what if." - What if the Bolsheviks had not taken over? Wouldn't things have been better? I don't know and I don't particularly like to work with those kind of speculative, counter-factual questions.

I guess Lenin's revolutionary achievements are seen in a somewhat less flattering light when you realise that the Tsar was already pretty much gone by the time of the October Revolution and what the Bolsheviks overthrew was the Provisional Government.

But what many people have most difficulty with is the orders Lenin gave during the Civil War to suppress farmers and rich landowners ("bloodsuckers") who acted against the Red Army and tried to withhold supplies from the cities- the so-called Hanging Order.

In this he suggested that large numbers of civilians be publicly executed. Deliberate attacks on the civilian population are usually seen as war crimes.

      Source:    Telegram to the Penza Gubernia Executive Committee of the Soviets in J. Brooks and G. Chernyavskiy's, p.77, Lenin and the Making of the Soviet State: A Brief History with Documents (2007). Bedford/St Martin’s: Boston and New York: p.77

My history textbooks frame him as a war-monger and mass-murderer.

Where are you getting your textbooks?