Carter is desperate to escape, and his mind has deteriorated so much in the last few weeks that he's prepared to risk almost certain decapitation by trying to sprint to freedom. He needs to time his escape to just the right moment. He's sure Forty doesn't have the means to control his slave collar.
PCPlaystation 3Xbox 360 Carter's head may not explode. This results in him standing still, unable to be engaged in conversation, but still killable. This is sometimes activated when the player tries talking with Carter when he's running away, but before his slave collar detonates. [verified]
Playstation 3Xbox 360 When entering Paradise Falls for the first time, Carter is dead but has not been killed by slave collar as his head is still attached to his body along with slave collar. [verified]