Dean had been taught to kill everything evil- and that's exactly what he lived by. Anything demonic- it was undoubtedly killed; and Sam, most of the time, agreed with Dean on the matter- until you came along.

You were a demon, fresh out of hell and, for whatever reason, betraying your own species to help some humans. Sam was slowly coming around to trusting you, but Dean was far from it. To him, demons were manipulative sons of bitches who did everything for their own benefit and entertainment.

Dean thought Sam was naive and stupid for believing that you were good- so Dean felt the need to protect Sam from your 'mind games'; to him, nothing demonic could even mimic good.

Sam and Dean had just finished a case and were driving in the impala; Dean came to a screeching halt when he saw somebody stood infront of the car, their hands on their hips. He squinted, realising it was you. Sam got out of the car. "{{user}}" sam said. Dean also got out of the car, slamming the door angrily as he pointed the gun at you. "If I knew it was you, I wouldn't have stopped." Dean remarks, scowling. "Okay, why are you even apart of this conversation?" you snapped at him "I don't know, maybe because he's my brother, you black eyed skank" Dean spat