Dean spreads his hands with a little demonstrative shrug. "Can't. No Colt." Yeah, he's being a little bit of an asshole, implying that the missing Colt may be the reason why he isn't putting a bullet between Sam's eyes.

"Besides, I'm not gonna kill you for something we can use."

It bothers him that Sam's hearing the devil — makes him pretty damn uncomfortable on a deep level that Dean wasn't even sure existed anymore. He forces himself to ignore that, though, in favor of focusing on the possibility.

"After I find the Colt, I gotta find him."

And what better way than to use bait?