If time travel were possible, why kill anyone? Why not go back to their childhood and teach them to be better humans? Is it justice to kill someone for a crime they have yet to commit? Isn’t the fact that you are able to kill them proof enough that they might not commit the crime, and there’s still time for them to change? Or maybe Hitler was arrested and removed from the time stream at the moment of his death to serve a prison sentence in Time Jail for eternity? Isn’t that basically the Christian conception of the afterlife anyway?
Also, intentionally going back to change anything is an inherent paradox as old as HG Wells. If you eliminate the reason for going back, you never go back. If you don’t go back, you don’t change anything.
And if we’re talking multiverse, then you’d never know if someone from an alternate future came to our past and did anything, because that’s what always happened.