If you go back in time and kill your former self, would it be suicide or murder? What if you went forward in time and killed your future self?
It's not clear that you can go back in time and kill your former self: a former self that doesn't get killed seems required for your presense at his side, contemplating the crime. This is a version of the grandfather paradox (you can travel back and talk to your own grandfather, but not kill him); though I should say that there are some (not me!) who hold that this type of thing is in fact made possible by a multi-verse interpretation of quantum mechanics. On the other hand, I don't see any impediments to using time-travel to kill off the final stage of yourself. (This needn't be in the relative chronological future if your life ends on a time-traveling adventure.) Whether it's suicide of merely murder seems purely terminological. It's "suicide" in one sense (you're done in by one of your own temporal stages), but not another (your final stage doesn't do itself in).