Was Buck justified in killing Spitz in The Call of the Wild?

Buck kills Spitz at the end of chapter three, and it is the culmination of several previous fights. Whether or not Buck was justified is up to individual readers, and it largely depends on a reader's opinion of killing in general. If killing is never okay, then Buck wasn't justified no matter what the reason.
I suppose Buck could have tried to run away or showed that he wasn't trying to be the dominant dog. Perhaps that would have ended Spitz's antagonism. On the other hand, a reader could say that Buck was justified because he had no other alternative. Spitz put Buck in a kill-or-be-killed situation, and it was clear that Spitz would keep trying until he was successful or dead. Buck killed Spitz in self-defense, and I think that is a fairly justifiable reason.