Unless he's about to be locked in a cell, chained to a wall, with no hope of ever having magic or seeing the MC again, there is no reason to ever leave this guy alive. He tried to destroy the world, nearly killed all of your friends right in front of you, and if I remember right has killed some of your friends in the past. Years of fighting against the same guy only for it to be a "spare" the enemy ending? That's disappointing.
Also, please don't drop this manga literally one chapter after you said you'd be going full time with it, the series is fairly popular it's just the fact this series has been nothing but straight fighting for three years, including this single battle that took over a year and a half, that people just want it to move past it.