Dean's eyes went wide when he heard something that sounded more like his Sammy, but then it was gone. Either this guy had multiple personalities, he was possessed, or some other freaky shit was going on...but Dean didn't have time to evaluate the thing that looked like his brother's lack of sanity.
"I've got no problem with killing you," He said, pulling out his gun and pointing it at his brother's head. "I never thought that you could be a monster like this, Sam, not under any circumstances, but I can see now that I was wrong. Thank God I kept my brother from that yellow eyed bastard."