That is an even worse version of a sucker punch. This should be prosecuted at another type of court.
Imagine amassing, within the course of one game, such hatred for another human being - during a cosmically meaningless leisure activity, no less - that you would say consequences be damned and unload a vicious and completely extracurricular sucker-punch elbow on that person, knowing it could seriously or even permanently injure them.
Part of me worries there is someone like that so enslaved to his own hotblooded temper running around freely. Another part of me worries it might be worse; i.e., this is just how this guy operates and this meant almost nothing to him. Either way he ought to do a couple nights in the pen and perhaps get a nice Full Metal Jacket blanket party.
Just as culpable are the parents. If you've raised a creature capable of such casual cruelty to others, you've made a few serious missteps in your role as disciplinarian.