First of all, if you're going to talk about killing MILLIONS of people, you're pretty much limited to the twentieth century, or maybe a couple of the biggest conflicts of the nineteenth, like the Napoleonic Wars. Wars before that simply didn't kill that many people, nor did anything else that was the result of human action.
The thing is that every war has two sides, and each side has its reasons for fighting that it holds to be valid. The leadership of either side will be regarded as heroes by the side they were fighting for and as villains by those who identify with their opponents.
Abraham Lincoln has -- to my mind, rather bizarrely -- been thrown out as a candidate here. He certainly was not responsible for the South choosing to secede rather than accept the results of a legally-conducted election. He did decide to prosecute the war to maintain the Union, and did so very aggressively, indeed ruthlessly, so I suppose you could say he carried responsibility for the deaths caused by it. On the other hand, he also felt that allowing the south to secede would leave an American continent divided between two potentially hostile powers that would inevitably fight war after war over the coming generations, killing many many more. So did he cause all the deaths of the war (about 365,000 between both sides), or did he prevent millions of deaths more that would have occurred in future wars?
Likewise with Truman - he is generally well-thought-of as a president, though not to the point of making him a "hero." Dropping atomic bombs on Nagasaki and Hiroshima killed somewhere between 150-250,000 people. Truman believed (rightly or wrongly) that many more Japanese than that would have been killed in an invasion, not to mention American casualties. So again -- do we think of him as the person who killed so many, or as the person who prevented the deaths of many more?
War is full of choices like that, and again, it takes two sides to make a war. Posterity usually gives its approval to leaders that people feel made the best choices they could under the circumstances.