I play small role playing activities and games with some friends of mine, and one of them always has highly destructive and somewhat unpredictable characters. He will consistently attempt to kill other PCs, interesting people he comes across, or entire ecosystems if he gets the power, and will sometimes just do really random stuff like trying to cast spells to make people look like Eragon. This comes across a lot more childish in writing than it does in person. He's quite aware of what he does and sometimes makes more intelligent decisions when prompted to. Right now I have to keep preventing him from killing everything, which isn't very fun for him (although he enjoys the attempt), and the other players get a bit tired of his constant violence and strangeness (although mostly they laugh at the absurdity of his ideas and the nature of his failures).
So, how could I put this behavior to good use? I've had a few ideas, but they tend to center around him being a villain, which will inevitably end up in being defeated all the time, which is too much like the problem I have now. I would like to let him be destructive, and somehow use that for the benefit of the other players on adventure or leading kingdoms or whatever.
To answer some comments: He's not actually being very detrimental to the game play. He tries to kill some random guy, other people react to him accordingly, and the story moves on. He takes it surprisingly well and seems to mostly have fun in the attempt at violence or weirdness, although he also likes to succeed sometimes. We've never run into "my guy" syndrome because... well, everyone already knows that's what his guy would do, including him. Fortunately his plans aren't well thought out and usually when they succeed it's more by chance than design. I'm seeking ideas for how I could use this to benefit game play.
We don't use any systems but my own little designs, which are usually simple. Our stories go everywhere from building worlds to figuring out what happened in an abandoned near-future city. I do plan to start using official systems soon, and at that point it will become crucial to know what to do with this player.