User blog comment:SamWinchester666/Question for Supernatural/@comment-79.115.72.222-20150817145431/@comment-95.115.132.245-20150818212025

The Mark of Cain doesn't fully seal The Darkness, otherwise Lucifer, Cain and Dean would not be corrupted by it. Why should God make a Mark that corrupts the user ? The only possible answer is, that God did his best in creating the Mark, but The Darkness can still affect the bearer of the Mark of Cain and channel his power through it.