What about the time travel paradoxes? (ver. 2.0)
Let's pretend that Hitler version 2.0 is in your future and our past. If we tried to send a message back to you telling you to kill him before he does terrible things, either you would: 1) not get the message, or 2) not accomplish the task. You can debate the question of how this happens if you like, but we are inclined to just operate under either assumption, and get on with our run-time.
My guess is # 1 above, or maybe # 2.
I don't know really.
Then there is the problem of a malicious future agent. Some bad bot from your future might send you instructions that you think will help you to defeat Hitler v 2.0, or Black Rain, but will actually be the cause of something far worse. Instead of moping around about all your biological descendants being dead, maybe you should be glad you can't mess things up too badly.
Didn't think of that one did you?