I wouldn't really say he's much of a loose cannon. Gantz tells him to do something and he does it. That doesn't make him crazy, it just makes him the perfect soldier.... and a little pyschotic.
Which is a bad thing. :P He'll kill whatever Gantz tells him to. (Though, I suppose the whole 'wanting to come back to Gantz' thing is a little crazy.)
He's quite good at it though. Unlike Nishijo (that eighth grader that died early on) he doesn't hide and use his teammates as bait. Instead, he dives right into the action, and tries to complete the mission on his own. This means less work for everyone else, and also less danger for everyone else.
So, there's pros and cons for him being alive and pros and cons for him being dead. Alive, he's helping the rest of the Gantz team, even though that's not his intention. He's also a significant threat to the Gantz team as he'd turn on them the moment Gantz ordered him to. Dead, he can't "help" anyone, but at least he's not a threat anymore.