User blog comment:Twilight Despair 5/The Darkness, God, Death, The Pagans/@comment-3045936-20150625000522

Possible reason why Death couldn't kill the Mark bearer. Like Rowena said to Charlie magic especially curses need balance cure one another needs to be cast or made. I think as the Mark is the first curse, it plays into the Natural Order. And as Death's power despite being far classes above even the Archangels, his powers are restricted to the natural flow of the Natural Order. So removing (undoing) the curse might damage the natural order, which why he could remove it just didn't but couldn't kill the owner. As for killing God, and not being able to completely separate parts of souls. I think the possibility of God's death doesn't hurt the natural order, as he is a living entity just a beyond understanding powerful one. His power just can keep death away forever, if he wants to.

But, destroying souls as energy cannot or shouldn't be destroyed as they too flow in the Natural Order. And destroying one, might damage the order to the point it might collapse.