I think it was a direct homage to the closing credits of Silence of the Lambs, where Hannibal Lecter does exactly the same thing.
Very foreboding symbolism too, if that's what the director was going for. Like Lecter, evil Tony is out of the box, turning his repressed anger outward, destroying everybody around him and feeling very very good about it.
I guess Dr. Melfi cured T. She told him Depression is Rage turned inward, and tried to see if he would consider Anger Management therapy. Tony has a better idea: much stronger endorphins to be had by directing the Rage outward.