User blog comment:Gcheung28/Supernatural Q&A/@comment-95.115.164.131-20150711144940

If souls can't be destroyed, what happens with the soul of an angel turned human if the angel regains his grace again (Like Castiel) ?

While he beared the Mark of Cain, Lucifer was indestructible. So why did Lucifer gave the Mark  to Cain instead of just copied it like Cain copied his Mark to Dean ? That seems to be a major disadvantage in his was against heaven.