People need to lighten up.
It's the whole 'Natural Born Killers' argument. Stone (despite being a self-righteous ****) said "I'm not promoting violence, or trying to make it 'hip'. I'm showing the irony of the media that take these things, such as serial killers, rapes or kidnappings, simply to sensationalise them and make a profit".
The thing I get from this is that kids nowadays that are the murderers, who carry knives with intent of stabbing someone, some of them are barely into double digits.
I think it's got potential to be a very, very good picture, however as Pete said, it comes across extremely set up, as if you were like "here, hold this", "uhh, okay?", and as Mr Not Spanish said, it really isn't executed very well.
I can see that it was probably quite a difficult shot to enact, simply with the rush you had to do with trying to get him to stand still, get the light correct, etc.
I think, I'd probably shoot it against a brick wall. Dying light, heavy off-camera flash, shot from a 45 degree angle so the flash's shadow is powerful against the brickwork. Leaned up against the brick wall, one foot up, the other down, knife hanging down, more natural (or even give him a toy or something and photoshop the knife in later so as to look more natural), and do away with the blood. The blood just doesn't fit and it implies he's killed someone.
I think the message might be better received had the blood not been put in.
I think this shot has potential. Ignore the naysayers. No one had a problem in 'City of God' with kids not even 5 years old running around killing each other, nor does anyone have a problem with Diane Arbus' 'Child with Toy Hand Grenade in Central Park'.