Not to mention the fact that Ben "couldn't" kill him even when he was right in front of him for some reason.
I'm going for one of two reasons:
1. They are each others constants.
2. They simply can't, as the rules of time travel indicate that you can't alter anything. So killing one another would ultimately change something, which would probably be anti-beneficial (lol at my lack of remembering words) in some way. Maybe similar to how Michael couldn't be killed (although I can't remember if that was explained or not).