After tGS I am starting to think Moridin is playing to win the Last Battle, not for the Dark One, but for himself. Moridin knows very well that the DO escaping means an end to himself, so why would he support the DO in this cause? He also seems to have no regard for the other Chosen, other then how they influence his plans; this seems slightly disloyal to the DO's cause. Finally (I can't remember which book it is now) when Moridin speaks of the game with the fisher, he speaks of playing both sides. If he is "playing" both sides, then maybe he is on his own side.

It just seems odd to me that he would knowingly bring about his own destruction, where as if he lets Rand seal the DO's prison then kills Rand he has a chance to rule everything and outshine his eternal rival.

I don't know if I believe this myself, but its a thought I had and wanted to see what others think.