Honestly, "Survivor Alien" doesn't equate to "Betrays Town"... he sides with the winning team, and currently Town is doing rather well, with 3 dopps versus 10 Townies/Aliens.
Besides, even if he is a Survivor and not a Doppseeker, lynching him would be a waste when he's already risked his skin revealing his ID to Dopps and Townies. Besides, killing a suspected Survivor is killing a prospective Team Member, whereas we should be lynching Dopps.
Logically, I suspect that his only motive in revealing his ID is as a Dopp Seeker, to work with the Town, snag a Dopp and win, before Dopps come down on him. He has little other sensible motive, and working with the Agent could rid us of another Dopp.
He's a Liability to Dopplegangers, not the town. A sensible Townie wouldn't ignore that fact.
EDIT: I don't mean to get you guys coming down on me now, but honestly, listen to reason. If Panda, Inaluct, and Webadict keep pushing to kill someone who is risking their skin for the town, has no plausible motive to betray us (it would guarantee his death either way), and generally keep pushing to get a Town Ally killed rather than seeking out Dopplegangers, you'll have earned my unshakable suspicion as enemies of the town.
EDIT EDIT: I know I'm a bit scatterbrained here, but I hope this makes sense to you guys. Don't kill him. Let the Agent check him out tonight. That gives us a way to kill Dopples easily, protects the Agent, and gives us a way to verify Webadict's honesty about being contacted, OR gives us a Liar or a Doppleganger to lynch tomorrow, and we'll be down to 2.