SPitz and Buck fight because they are both enemies. SPitz tried to steal Buck's bed and where he stayed, so Buck was furiously mad at him. This started a series of chaos fights and In the end Buck kills SPitz. This shows that Spitz is not fit to survive the wild if someone like Buck who is new to this type of scene can beat him in a fight.
One morning Dolly wakes up and she has rabes! She goes wildly mad and goes after Buck. She wants to kill everyone because she is infected by this horrible disease. Francois however takes his axe and throws it at her head which in doing so, kills her.
Buck is still alive with his new owner John Thornton. John had told Hal, CHarles, and Mercedes to not go on the ice bridge, but they did so anyway with all the dogs. This was a terrible mistake because they all died falling down to their own death. This shows how John Thornton and Buck are both still alive and what the story tells.