How does Buck feel about himself when he is hunting and killing?

Hunting is natural to Buck and brings him great satisfaction. He wants to kill, and he is proud of his accomplishments.

He began to sleep out at night, staying away from camp for days at a time; and once he crossed the divide at the head of the creek and went down into the land of timber and streams. There he wandered for a week, seeking vainly for fresh sign of the wild brother, killing his meat as he travelled and travelling with the long, easy lope that seems never to tire. He fished for salmon in a broad stream that emptied somewhere into the sea, and by this stream he killed a large black bear, blinded by the mosquitoes while likewise fishing, and raging through the forest helpless and terrible. Even so, it was a hard fight, and it aroused the last latent remnants of Buck’s ferocity. And two days later, when he returned to his kill and found a dozen wolverenes quarrelling over the spoil, he scattered them like chaff; and those that fled left two behind who would quarrel no more.

The blood-longing became stronger than ever before. He was a killer, a thing that preyed, living on the things that lived, unaided, alone, by virtue of his own strength and prowess, surviving triumphantly in a hostile environment where only the strong survived. Because of all this he became possessed of a great pride in himself, which communicated itself like a contagion to his physical being.