What is the source of pride Buck feels after he attacks the Yeehats?

From the text:

At times, when he paused to contemplate the carcasses of the Yeehats, he forgot the pain of it; and at such times he was aware of a great pride in himself--a pride greater than any he had yet experienced. He had killed man, the noblest game of all, and he had killed in the face of the law of club and fang.