1) It was quite necessary not only for Buck, but also for others struggling to survive in the wild North. If he hadn't killed Spitz it it could have been very dangerous for others because he was very aggressive due to the hunger. Even though he was the leader, blinded by hunger he could do anything to survive, forgetting about the rest of dogs.
2) If he hadn't died he could have get many other innocent lives, like he had done to Curley. This character yielded to the external conflict of the harsh wild conditions, and his instings were much more powerful than his sense of justice so he could have bring harm until he reached his own safety.
