10 Dumbest Decisions Made By Supernatural Characters

Despite the constant warning from Cain about the Mark and the burden that comes along with it, Dean was adamant about taking the Mark of Cain for himself so he could have the strength in killing Abaddon with the First Blade - which could only be used by the one who held The Mark.

The Mark, as an aside, is used as a key to lock the Darkness away and whoever bears the deadly scar becomes the ultimate killer. While Dean was successful in killing Abaddon, The Mark turned Dean into a ruthless, aggressive killer as he was slowly becoming influenced by the Prince of Hell's power and couldn't fight off the evil that raged inside him from The Mark.

Dean's decision to accept The Mark proved to be one of the worst things that not only happened to Dean himself but also to those around him. After almost killing Sam and Castiel (plus many, many more), indirectly causing Charlie's death, killing an innocent Cyrus Styne, turning himself into a demon and unleashing the Darkness after killing Death himself, Dean's choice to bear The Mark of Cain caused a tumultuous domino effect on every person in the universe.