Well, the problem, in my eyes, with killing someone for crimes they've not yet committed is that you cannot be sure that they were actually going to commit those crimes. Given a means of time travel, that's no longer necessarily true, since you can observe the crime being committed and then prevent it from ever having occurred. I mean, punishment after the fact is not really ideal (it seems generally agreed that to prevent a crime in the first place is preferable to punishing it), but it's the only practical tool available given the rather harsh limitations on prescience.
Of course, you'd need some sort of extra-temporal records to document everything so that people aren't just travelling through time and murdering others on whims, claiming, "Oh, this person was going to commit genocide!" as an invulnerable defense. Not that many people would do that, but hell, even one would be enough of a problem. That's probably the primary objection I'd raise to assassinating pre-Nazi Hitler. That and the various arguments that could be made about actually making the world worse what with somebody more competent replacing him or technology not advancing due to a lack of such an effective stimulus or whatever, but those are pretty dubious, I've always thought.
EDIT: Although, I like Vector's answer better, since it's not merely morally neutral. I feel bad for not considering it before typing all that up.