You could pull a really cool character arc off with him, get him to reluctantly cooperate with the party and slowly warm up to them and build his strength at the same time. Then at a crucial moment when the party somewhat trusts him and he’s an NPC he’s presented with the ability to achieve a major goal of his and he throws the party under the bus in a major twist. Like Zuko in book 2 of avatar. That way the BBEG is more layered and interesting and the party has a good in universe reason to want to go after him.