He could just be a psychotic killer. Lots of adventurers have died lately so it’d be easy to hide a few murders in those numbersIs there really a benefit in doing this? Getting the loot from a competing carrier can't be THAT profitable for the risk, and luring out higher level monsters shouldn't be worth getting other people killed.
Best I can imagine is sniping the mage girl to join their party, but that's an absurdly roundabout way to do it.