Is it ever okay NOT to kill Hitler?

In honor of the recent Doctor Who episode 'Let's Kill Hitler', let's talk about the classic values-clarification type question: if you went back in time, would it be morally justifiable to kill Hitler?

Before we get into the timey-wimey aspects, let's look at the question. It's not just Hitler--- there have been loads of heads-of-state who have, like Hitler, given orders that lead to the deaths of large numbers of innocent people. Joe Stalin and Mousey Tougue (all right, Mao Zedong) both killed many more millions than Hitler.

Even in earlier ages we had heads of state who caused mass death, though not on quite the scale of those who had both industrialized methods and far larger populations to work with. Caligula and Nero Caesar were both very notoriously violent emperors. Tiberius, though not as insane, was also prone to solve state problems by ordering numerous executions. Even more respected emperors like Claudius and Augustus had a number of wicked deeds to answer for.

But in spite of all the misdeeds of the Caesars, nowhere in the New Testament is it written 'render unto Caesar a sword to the heart'. Indeed the Caesars were as far as Christ and the early Church were concerned, the legitimate bearers of the sword of authority, to whom taxes ought to be paid.

One problem with the assassination of heads of state is that there is disagreement on the exact nature and quantity of evil deeds that a head of state must commit before being an acceptable assassination target. Abraham Lincoln, while alive, was blamed by many for the bloodiest war in American history to date.

The act of assassination does not get rid of the policies of the assassinated one. The policies of Lincoln and John F. Kennedy were carried out after their deaths, in part because their deaths made them martyrs which made it more difficult to argue against them. And even rulers seen by most as wicked during their lives may be considered martyrs should they die by assassination.

But this is a time-travel question, so we must get to the timey-wimey aspects. At what point would one kill Hitler or Stalin? At the very end of their historical life-span, it may be considered very justifiable, but does it do a sufficient amount of good to be worth the effort? All or nearly all of their victims are already dead.

If on the other hand you go back to ever earlier points in the historical career of your target, though you are perhaps saving more lives you are killing a much more innocent Hitler or Stalin. Go back to the career-beginning--- as when Hitler, a World War One veteran just released from service, made his first political speech--- and you will be killing a Hitler who has not yet been implicated in one wrongful death.

Can you justify killing a person based, not on what he did, but what he will do in the future? More precisely, based on what you assume he will do in the future? If you can change history by going back in time to kill Hitler, then history is not fixed. Which means that there is no certainty that the young Hitler before you will now go forward to have the career you remember from the history before you went back in time.

I cannot help but think that such a killing would be unlawful murder. It's somewhat similar to the eugenics movement claiming that certain persons because of the criminal history of their parents are doomed to lead a life of ever worsening crime. Even if that were true, it is still unjust to punish a person for crimes he will commit, but has not yet committed.

Another time-travel question: perhaps your time-travel machine is not good enough to get you past layers of security that any head of state would have. Suppose it is also not that precise about the time of arrival. You land in the home town of Adolf Hitler, walk to the home that Hitler grew up in, and find a small boy crying in the street because his drunken father is beating his mother again. That boy is Adolf Hitler. The gun is in your hand. Do you pull the trigger?