Just a thought: if Morridin helped Rand, what is his motive? If he really is a true personification of Death (Though I suppose it seems unlikely) instead of a Rebirthing, he would be a slave (DO is the Lord of the Grave). In the short instance that we met him, (CoS) he seemed rather strong willed. If he is a slave, I'm sure he would chafe under bondage. Perhaps he believes that if Rand seals away the DO for eternity, he'll (Morridin) be free to be a more stable, cyclic entity, one more in harmony with a birth entity. Dunno, though.