Could Dean Turn Evil?

Could Dean possibly turn evil or is he already? I was just thinking about this today. A lot of people would say that it's not possible for Dean to turn evil because they think he's on a mission from God. I don't think God had anything to do with Dean getting out of hell. Like I said before, is Dean still under his contract. How did he get out of it? I doubt Castiel pulled him out and his deal just went away. How do we know that Dean didn't make another deal in hell? Maybe he made a deal with Lilith. I just think of how Dean treated Sam all season long and I don't get it. I could understand Dean being afraid of Sam turning evil by using his powers, but Sam still was the same Sam he's always been, so why didn't Dean want Sam using those powers? Why didn't Dean do anything when he found out that Sam was talking to Ruby again and did Dean sleep like the dead, that he didn't know that Sam was slipping out and doing what he was doing.
I think about when John made the deal for Dean. Who knows what that entailed? It looked like the YED is the one who took over the Reaper and pushed Dean's soul back to his body, who knows what else he did to Dean. Dean said that he didn't feel right. Was all of Dean's anger and agression in season 2 because of the father or something the YED did to him.
Both Sam and Dean have died. Dean was dying in the hospital and we saw what happened with him, but we never saw where Sam went or if he talked to a Reaper. Maybe Sam was able to come back to his body on his own or something else had him go back there.
I just think there is a little possiblity that Dean could turn evil or has evil inside him. He went to hell after all. He tortured and he enjoyed it, so go figure that. There has got to be something inside Dean. Why didn't Dean care about the seals breaking? Maybe that's why Dean was having nightmares, not about what happened in hell, but for the new deal he's made and what could happen. Who knows?
My theories are crazy, but they could be possible. Who knows what Kripke and Co are doing? They could be trying to make us think that Sam is evil or going evil, when it's really Dean. That would be a big surprise, if that did happen because most people would never expect it. People expected Sam to turn evil because Kripke and Co kept leading people on and especially in season 4. They purposely didn't let you know what Sam was doing and when you did find out, they made it look like Sam was dark and evil.
I just love to consider all possiblities. Who knows, the YED could be in Bobby right now for all we know? lol