Michael Jackson's songs...

Did he write the music or just the lyrics?

Quincy jones was VERY influential in the writing of MJ's music, but I think it's safe to say that jackson himself wrote quite a bit of it.

I think it's also pretty safe to say he molests adolecent boys.

Possibly he wrote SOME of the words, as he doesn't play any instrument (he has been photographed pretending to play piano), but mostly he's known to only accept/record songs if he gets a "piece of the pie".

Not an uncommon practice in Big Time Music, actually.

I always thought he wrote the bass line to billy jean. Now that I think about it, how the heck did he do that?

As a long time MJ fan (of the music), I've read that he would often hear an entire arrangement in his head and convey it to the musicians via his voice. He'd hum a bass line, or a guitar part, and Quincy and the crew would do their best to turn his musical vision into reality. Quite amazing, though I doubt this method would work for too many 'non-instrumentalist' songwriters who don't have access to (or budget for) the best producers and musicians in the industry.