In The Call of the Wild, Buck wanted to be the leader and didn't always do what Spitz wanted. What was the result of this?

Buck fights and kills Spitz and takes over as lead sled dog.

Spitz is the leader of the sled dog team, and Buck grows to hate him. He refuses to give in to Spitz, eventually killing him and taking his place as the leader.

One day, Buck sees Spitz fighting Curly.  The brutality of the fight upsets Buck.  Once Spitz attacks the dog, all of the others attack her too with no mercy.

So that...

Buck fights and kills Spitz and takes over as lead sled dog.

Spitz is the leader of the sled dog team, and Buck grows to hate him. He refuses to give in to Spitz, eventually killing him and taking his place as the leader.

One day, Buck sees Spitz fighting Curly.  The brutality of the fight upsets Buck.  Once Spitz attacks the dog, all of the others attack her too with no mercy.

So that was the way. No fair play. Once down, that was the end of you. Well, he would see to it that he never went down. Spitz ran out his tongue and laughed again, and from that moment Buck hated him with a bitter and deathless hatred. (Ch. 2)

Buck is determined not to go down.  He is learning the ways of the wild, and the Law of Club and Fang.  Basically, that means that whoever is strongest is in charge.  The humans have the club, and the dogs have the fang.

Spitz knows that Buck is a “dangerous rival” and is careful of him, always showing aggression toward him and never letting his guard down.  Buck also does not back down.  A conflict between the two of them is inevitable, and they both know that when they fight it will be a fight to the death. François and Perrault know this too, but François believes that Buck is the fiercer one.

There was no hope for him. Buck was inexorable. Mercy was a thing reserved for gentler climes. He manoeuvred for the final rush. The circle had tightened till he could feel the breaths of the huskies on his flanks. (Ch. 3)

Buck takes Spitz’s place as lead dog when it is time to strap everyone in.  The men are surprised. Perrault tried to make Sol-leks lead dog, but Buck wouldn’t have it.  It was his spot.  He earned it.