User blog comment:Kajune/SPN's redeemed antagonists/@comment-34326521-20160601094609/@comment-34326521-20160602205240

Sure, Lucifer could have killed him. But Gadreel allowed him in.

SeraphLucifer, angels may be naive, but every single time he tried to get one or more angels to trust him, they didn't. Only Gadreel was tricked, because he trusted Lucifer. Castiel consented to the possession because he had no choice. Either Lucifer was telling the truth, and he really could defeat the Darkness, or he was lying, and then it wouldn't really matter.