A typical case of "You moron! You both have the same goal! You could be allies, but instead your first move is to murder someone!"
To be honest, I don't think it's the attempted murder what makes him a bad guy. If you feared that the Master of the Ring is going to brainwash your beloved into a mindless slave, and you see he's already surrounded by a small army of demons, yeah, I can see why someone would choose a "better safe than sorry" approach. But the moment he learned from Asmodeus that Ritsu is freeing the demons, he should have stopped. He's so obsessed with trying to prove he's the "worthy" master of the ring ("The value of the ring is being lowered"... Wait, that's what worries you?) that he's forgotten where his obsession came from.
Who bets the MC will remind him of that childhood promise and there will be a heel-face turn?