Why does Buck feel contempt for Hal and Charles?
Although Buck has had to deal with some cruel and violent owners in Jack London's Call of the Wild, he is an intelligent canine capable of recognizing the different actions of humans. Although Buck's previous masters whipped and beat him when they felt necessary, they also ran a taut ship: The dogs made good time on the trail, they were fed sufficiently well, and they were tended to when injuries or the cold affected them. Hal and Charles were not like his other masters. They were new to the area, did not handle the dogs well, and compromised the safety of them all with excessively heavy loads. Buck could sense that these new owners were not equipped to handle the hardships of the trail, and he must have known that his life would be a short one if he remained.
I do not think there is any place where it actually says "Buck felt contempt for these guys and Mercedes because..." However, I think that what you are probably looking for is the idea that Buck thinks they are stupid because they don't know how to do things right on the trail.
Most specifically, they are really bad at setting up and breaking down their camps. It takes the a long, long time to do either of these things and so the only make it about half of the distance that they ought to in any given day. Buck had also worried about them at first for similar reasons -- they had way too much stuff and didn't realize it.
So I think he feels contempt because they don't know what they are doing.