Time travel makes ethics difficult.
Limited by a lack of knowledge of what the future can bring we are free to act in a way that makes sense. You do the best you can in the time you are given. If you can stop a wrong you stop it, if you can do a right you do it. But time travel confuses it all because we can see the world that past wrongs, and there are a lot of them, created.
Were you born after 1939? (I was.) No matter who you are, not matter where you were born, the odds of you being born in a world without Hitler are effectively zero. They're not quite zero, just damn close.
And even for those who would have been born, the lives they would have lived in a world without Hitler would be completely different. Which means that the people they are today would never exist.
Kill Hitler and you wipe out the entire population of present day earth. Most you destroy entirely. Not just kill them but erase them from history. Those that aren't erased are replaced by bodysnatcheresque copies of themselves.
So it becomes complicated. Murder seven billion people and you prevent the deadliest conflict in all of human history (of course, who knows what other conflicts might arise) and birth a world without the Holocaust, without the only use of nuclear weapons against actual people in our history.
One can make a story that argues that you should change the past, I have some scenes from one such story in my head. One can make a story that you shouldn't. It's why the Doctor doesn't kill Hitler, some things are fixed. Change them and you change everything, and his companions would never have existed.
As a human being you deal with what's in front of you, not what may one day be. If you find yourself stranded in the past then you fight to make it right and history can go fuck itself. You don't let the people in front of you die just so your world can maybe one day come to be.
But if you're not stuck, if your future world is just car ride away (88 miles per hour) then what do you do? Do you destroy that world and all those in it in favor of some other world? Do you stop the Holocaust then?
September 11th 2001 also comes to mind. It's more recent, but prevent those attacks and there are still plenty of people alive today (all of them children) you very definitely kill.
What if you're not in the past at all? What if there's a big red button in front of you, and if you push it Hitler dies peacefully in his sleep the night before the day in which his evil contribution to world history would have become irreversible.
You're still here in 2012, you can see the people you'll be condemning, the world you'll be destroying, yourself as well, will be wiped out. The people you would save are already dead. The people you would exterminate are very much alive. Push the button, stop World War II and the Holocaust and all associated ills. The only cost is the lives of everyone in your timeline.
Do you push it?
It is, of course, theology that brings this question to mind. I'm trying to catch up on some reading and the question of why god, if there is a god, allows the world to suck.
I am the product of an abusive relationship. I've never had any reason to believe that the relationship wasn't abusive when I was conceived, so presumably a world without abusive relationships is a world without me. If God hadn't allowed the world to suck when I was conceived I never would have been born.
That doesn't make me want there to be abusive relationships in the here and now or in the future, even though stopping them would mean that people like me wouldn't be born. The future is the land of potential and you don't allow present evil in hopes of potential good down the line.
But if the God many people seem to want had been around a few decades ago, I would not be here now. And since some say God is timeless it would mean that all of history is what's in front of it. The decision to allow the world to suck was made while I wasn't potential, I was actual. A decision to make it not suck would have meant killing me to bring some hypothetical world into being.
I follow no religion, I have no creed, but I have a personal stake in these arguments because the question of why the world is allowed to suck is inseparable from the question "Why are people like me allowed to exist," it's my existence the question is talking about, mine and everyone else's but mine is the one that makes it personal, and when you bring time travel into things, as people do when they assume that all knowing means that the future is what's in front of god or that god is timeless, or any of the various other assumptions that mean that god can see what will result from it's choices, ethics become complicated.
"Would you kill/stop Hilter," the problem of evil asks, "knowing that doing so would kill everyone on earth right now in one way or another and replace them by some currently hypothetical world that may be better or worse?" Theology asks difficult questions that I don't have much answer for.
I can answer that for a human being (the answer is "it depends") but for the kind of God people are talking about in such situations, I have no answer.
Except, perhaps, quantum physics. Have a multiverse where every possible permutation of existence goes down, and then sort it out in the afterlife. But in that case we're talking about a God that allows for much worse evils than the ones we have seen in this world, and the depths we've seen in this world are pretty deep. So, then, the problem of evil likely becomes amplified rather than answered.