Time travel is something that almost always doesn't work unless you invent an inherent logic in your universe to make it work. The basic problem is this: there are 2 basic routes, the deterministic universe (which does work, like in la jetee/12 monkeys) but usually isn't a lot of fun to read (or think about since it means we can't really have free will) and the infinite timelines solutions, where you go back and even by going back you create a new timeline which would not affect that timeline you just left (this is the one writers usually use, but it almost never makes sense). That's why my suggestion is to add something of your own which changes the basic logic. You could play around with the idea of dimensions (which could mean whatever you want it to mean). What if the was a "thing" that has the ability to destroy/kill/undo across ALL dimensions/timelines? (even if there is an infinite amount of them).
They were first hit by a spell that greatly compromised their powers, leaving them powerless to defend themselves. When they were killed, there was also a spell put on them where it's impossible for anyone else to save them through time travel.
Paradox. Suppose someone is killed 5 years ago. You go back in time 6 years and say to them, 'in one year, someone will try to kill you.' Now suppose that warning works and the person isn't killed. You would then have no reason to go back in time to warn them. If no warning is given, the person is killed. But if the person is killed, a warning would be given. The paradox kills the time traveler.
