He only became the Dark One to save his boy. He deserted the Ogre Wars like anyone one who was drafted to any war would. Who wants to be forced to go to war? He is evil but nevertheless because of his need to save his boy he felt aquiring power was the only way to do it. He was tricked by Zoso, the former Dark One to take his life and his place. He takes first borns because first borns go to war. Going on that theory if he takes all of FTL’s first borns no one will go to war for the ones seeking the first borns but go to Rumple and strike a deal for them.