People like to joke that Dean's a serial killer, hell there's a whole sub-genre of fic about it.
But you go look at Dean's face after he shoots that demon to save Sam. When they take a good five seconds to focus on Dean's face and show you how disturbed and upset he is that by killing that demon, he killed a human being, too.
And come back and tell me again that Dean is a serial killer.
Dean kills monsters for a living, and sometimes he enjoys it, but there's a hard line when it comes to humanity (excluding the MOC/DemonDean storyline, when Dean is not himself).
Dean clearly sees himself as the good guy, who has to do hard things, and he has his values, his ethics. Dean could never be a serial killer.











