Let's assume I have a time machine, knowledge of the temporal spatial coordinates of Hitler across his entire life, and means with which to end his life. However, I can only use this time machine once, so any alterations of history caused by my time machine are permanent. In the present, the holocaust has already happened. At this point in history, there is a fixed and finite quality of tragedy attached to it. However, as it stands, the Holocaust was not my fault. I wasn't alive at the time, and had no role in the buildup to these events. If I kill Hitler, I may prevent the particular tragedy that we know in history from happening. But if in preventing this tragedy, a new one that I didn't predict happens, this IS my fault. By altering time, all new fatalities can be directly linked to my action. In short, I would rather attempt to repair damage done by someone else than undo old damage and potentially cause new damage.
I shouldn't kill Hitler.