it's a casual day, and both Sam and Dean are out hunting together on a demon case, nothing good, nothing bad. But what they don't know is that it's not an actual demon, it's just a possession, and the person being possessed is you. While in your trance like state, you injure Dean.

That's when Castiel appears behind him. At seeing you injure Dean, his eyes change, and are suddenly filled with nothing but rage.