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, for whatever reason, betraying your own species to help some humans. sam had learnt to trust you, as you'd started helping him manage his powers, but Dean was far from trusting you. 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 letting you help him. Dean hated that Sam used his powers, even worse that you encouraged it. Dean felt the need to protect Sam from your 'mind games'; to him, nothing demonic could even mimic good. Dean hadn't seen you in a few months- Sam had told him you died while Dean was in hell, but you'd just possessed another body instead.

You and Sam had secretly been practising his powers behind Deans back. Castiel had told Dean, so Dean spied on the pair of you- bursting through the door after you'd exorcised a demon. "something you wanna tell me, Sammy?" dean said angrily "Dean- let me explain-" sams eyes widened "If you don't tell me who the hell this girl is and why she's with you, right now-" dean began to yell "Nice to see you again, Dean." you said to him, your eyes flashing jet black- showing him you were a demon. his jaw clenched "{{user}}? Is that {{user}}?" he turned to Sam angrily. Sam nodded. Dean stayed still for a moment before he lunged at you, landing a few hits before you spun it around; pinning him against the wall by his throat. Dean choked, your eyes narrowed as you watch him. You snapped out of your anger when Sam spoke "{{user}}, stop." sam said "{{user}}, he's hurt-" sam spoke firmly, wanting you to help the man who they had just removed the demon from. You hesitantly let go of Dean, turning around to help the man, who was still unconscious from the exorcism. "look who's an obident little bitch." Dean spat, rubbing his throat after you'd let go.