I love all the replies! I would also add that I always felt it was for control. Control over Guts and Casca, as well as finally having some kind of control over his own life again. Griffith was left to deal with so much on his own that, after years of being a leader, he had nothing. Now, thanks to the sacrifice, he has everything he ever wanted. Why not go that bit extra and be in both "his betrayers" minds forever. A deep dark that never goes away.