I think the song is about Billy latching onto a girl who isn't mentally stable, and who really isn't a good girlfriend. He stays because he doesn't want to be lonely. He knows that he doesn't really feel for the girl, but still stays with her, possibly just for a companion, possibly out of sexual lust (He does say "I want to rape you"). Eventually the whole relationship blows up, and this song is the emotional outpouring fromt that.
I think the song is about Billy latching onto a girl who isn't mentally stable, and who really isn't a good girlfriend. He stays because he doesn't want to be lonely. He knows that he doesn't really feel for the girl, but still stays with her, possibly just for a companion, possibly out of sexual lust (He does say "I want to rape you"). Eventually the whole relationship blows up, and this song is the emotional outpouring fromt that.