"You're saying my molecules go somewhere. If I'm not me then what difference does it make where I go?" He wasn't being argumentative, he was trying to figure out what the point of it all was--just like every other human on the planet. Billy hadn't stuck around in college long enough for a philosophy class, so he didn't know Descartes from Skinner, he just operated on his own.
He'd have a hard time answering if someone asked him whether or not his continued existence was his purpose, though. There had been times when his existence had sucked so much he couldn't really see the logic of dealing with it. Aura had been around for those times. Before he had met her. (What happened to sense?)
You could tell he was thinking about something that wasn't in the car with them, because he blinked hard to pull himself out of it.