Xander cursed Amy under his breath, as he was tugged back and forth like some prize in a demented tug of war. He had just wanted to get back at Cordelia, maybe salve his bruised self-conscience, not be turned into catnip for the entire male population of Sunnydale, or end up being fought over, like a chew toy, between a vampire and a werewolf.
Yet, if he was going for the lesser of two evils here - Oz was his man. Excessive hairiness a few days a month versus permanent death (or undeath) was a no-brainer. So, it was instinctual for him to back toward Oz, as Angelus shifted into game face and growled at them.
However, Xander realized he might have miscalculated when his short friend grabbed him by the neck, and growled back. Seeing Angelus smile while in game face was a scary thing - but, not near as scary as the wood he was suddenly sporting when he realized that instead of continuing to fight over him, his captors decided that there might be enough of him to go around.