I want to try this. Do you think it would work to go on myspace, copy someone's profile exactly and start coming out of the closet and breaking up with their girlfriend/boyfriend? Also, would there be any legal repercussions?

First of all, that would probably not work.
I also doubt that it would ruin anyones life, and there could be legal issues depending on what exactly you did and how far they were willing to go to get you for it.

If everyone thought they was gay it wouldn't work?

You really think that a fake MySpace would make people think someone was gay? What about the real MySpace, that has all that persons friends already, and they can just say, "Hey ignore that other one it's fake" and everyone thinking someone is gay doesn't mean their life is ruined, that stupid

What about hacking their old one?

Hacking their old one would be alot more effective, although they could still just tell everyone their profile got hacked.

Ruining someone's life who you have met would be a lot easier to do because you could then make it a lot more personal.

It is really hard unless you've got a lot of time.

Consider this: http://www.totse.com/community/showthread.php?t=2153900

post their personal info on 4chan /thread.

http://www.foxnews.com/story/0,2933,312018,00.html