Unfortunately, where there is free speech, there will also be foolish speech.
Of course, and I’d die for that right. But there is a limit on free speech if it results in mass injury, for no reason. Crying “Fire” in a crowded theater is ok if there IS a fire. The song’s writer would get away with it in my mind if at the end the boy goes to jail or hears sirens coming or some such. This way a sick teen who sees this as his way out can see that such violent fantasies have serious real consequences.