Of course, just knock him out. It's not like he's a murderer or anything, and he definitely won't come back to attack you later!
Edit: Though, looking back at it, it doesn't look like he ever actually murders anyone on-panel. He talks about treating players and monsters "the same" though, and he directed twisting magic at the girl here with enough force to completely obliterate the giant robot's arm, so at the very least he's likely an attempted murderer, though from the monster comment I assume he just murders people regardless rather than just horribly maiming them.