How does Spitz from Call of the Wild by Jack London show his treachery?

When the wild Huskies attack, everybody (sled dog and man) pitch in the fight. Buck is right in the thick of things biting the neck of a Husky dog and tasting its blood. Meanwhile, Spitz uses this moment as an opportunity to continue his attack against Buck. He keeps lunging at Buck's throat even as Buck is fighting the other dogs.