This picture shows when Buck was being bought and the moment Buck's life would completely change.
”kill or be killed” This was the only way to survive.
Buck realized that in order to survive he must obey and respect the man in the man in the red sweater.
Buck and Spitz would always fight to prove to the pack that they were worthy enough to lead them through h enough journey.
Eventually it came to their last fight where Buck had no hesitations and killed Spitz and that was all it took to prove that he was worthy enough to survive and lead the pack.
Finally Buck had gotten what he wanted as lead dog of the pack after killing another dog for the first time.
In the end Buck ended up becoming a wild dog and joined a pack of dogs after killing a few humans when he realized that they had killed a man he cared for very much. Buck was now feared by many after he became a savage pack animal.