Before he sent her back to Crescia, he had already made up his mind on this matter. He was going to end everything and make sure to completely eradicate any and all threats that could lead to Evie's death. And once all of that has been completed, he was going to back off and leave quietly.
Gavrael already had all these planned out. When he found out about Galleous' existence, he had already planned to kill him by devouring all his dark magic. This was the only way to defeat someone who was hiding inside an ancient crystal. In the Under Lands, devouring someone's magic was one of the forbidden spells that Gavrael had picked up and learned on his own. He had tried it out once before, back when he was still in training on one low levelled monster. He had managed to suck out all of the monster's magic and energy dry and when he had succeeded, the monster had turned into nothing but a pile of bones and hair. He was going to do just that to Galleous!
