He is in love with King Ashura and "will do anything to get what he wants." Therefore, Taishaku-ten made a deal with him: he could have the King Ashura, but had to fulfill his wish of preventing Ashura's son from becoming a God of Destruction. This leads to the painful act of him eating King Ashura later, as King Ashura himself said this is the only way for him to have strength to rebel against the Emperor. Taishaku-ten then successfully killed the Emperor and got the title himself, becoming a tyrant just to fulfill one wish of the man he loved.
According to Taishaku-ten, living alone without your loved one is extremely painful, as he had always suffered being without Ashura-Ou.
