"Everyone thought I'd fallen on some broken glass by accident. But . . . I just couldn't stand myself anymore, so I went behind the amps with this piece of broken glass, having decided to cut my jugular vein. I just didn't have the guts, though . . . I was aiming for the vein, but I just couldn't make it. I cut up my chest instead."
Iggy Pop's explanation as to why he used to cut chunks out of his chest on stage means nothing. The Stooges singer used to get so strung out on drugs and alcohol before each show that he probably couldn't tell his jugular from his left nipple, or a shard of broken glass from a roll-on deodorant stick.
You've got to be wary of performers who cut themselves up on stage. There's a good chance they're indulging in cheap shock tactics to distract the audience's attention away from their crap music (e.g. Marilyn Manson). Then again, there might be more valid and meaningless rock 'n' roll reasons, like they've completely screwed up their brain (e.g. Iggy Pop) or were born without one (e.g. Sid Vicious).