Got ya being soft... Just kidding.
***Perhaps Hitler or Stalin, or other geo-political figures are not the best examples, here. How about lone madmen? How about the unibomber, or John Wayne Gacy, or Ted Bundy? If you could travel back in time to visit Bundy's mother, Why is the blood of their victims not on your hands simple because you're not the one holding the knife? Isn't allowing someone to die an immoral act, even if saving their lives means commiting a lesser atrocity yourself?***
Of course, if we are talking about a current atrocity, then we have every right to take action of self-defense or the defense of someone at the time of clear and present danger. However, if there is no clear and present danger, then we need to wait for the authorities. If we have information on such an act or potential act, we wait for the authorities. Vigilantism is wrong and an immoral act - especially if it is conducted against someone's parent. Afterall, it could be you that raised Ted Kaczynski.
***Why is the blood of their victims not on your hands simple because you're not the one holding the knife?***
Well, when talking about going back in time, you are talking in different terms then being at a point of clear and present danger. Now you are talking more in societal terms of fixing all that is wrong with society. Yes, you might be able to go back and fix a few incidents, but society is a system, a very large system, and you can't fix one cog in the system without affecting the other cog. Let's take 9/11 for example. As much as many people would like to go back in time and say surprise to those evil monsters the morning they awake at some motel (with a 357 magnum in their hands), the fact is that history has evolved to the point of mass terrorist events, and as a time traveller or clairvoyant you can't possibly prevent the inevitable. All you can do is weaken the immune system of that society that is approaching dangers times and, in a sense, prevent that society the opportunity to restore proper defenses against the 'diseases' of terrorism that would, if unchecked, destroy society.
Does that mean that if you have information that you are not forthcoming? Of course not. Why? Because one of the blessings of being in time is that the future is undecided (as far as we know), and that we cannot know what is bound to happen from a societal evolutionary standpoint. However, living in the future (like a God's point of view to a certain degree), we already know that just preventing 911 on 9/11 is not enough. If the necessary political and military aftermaths of the event were not taken - to the degree they were taken - then the overall situation only grows more ominous.
Like I said, I don't like thinking in terms of crimes and punishments, etc. The whole discussion strikes me as creepy. I wouldn't want to be in the situation, nor tell anyone how they should feel for those who were. When you talk about these kind of issues, its not just armchair philosophy, this is stuff that affects people's lives and, for me, its unsettling. I wish we lived in a perfect world where no evil exists. But, I fear such a world wouldn't be as perfect as the one that is evolving here.