If the paladin could time travel, why not just stop evil from ever being created at a conceptual level. Or why dosen't he go back in time and kill the first of each intelligent race to prevent evil from happening? As for the real question, assuming he can travel back in time and all he wants to do is stop one evil guy, he has presumably met this evil guy. Meeting said evil guy has changed his life in some way. If he kills the evil guy then he will not be the same as when he left, and may not have discovered time travel. If he can't time travel, how could he change the past? If he couldn't change the past, then he will discover time travel after all...
Ow, my head...
(edit: this was written before the other, similar post above. I did not steal the idea)