If this is genuine, then I am with the Guardsman all the way.
Al R, you seem to be mellowing too much in your old age, and maybe are forgetting how YOU felt when doing stag/guard. I may have done my guard duties twenty years ago, and while most were boring and tedious, it was not uncommon to be spat upon, called names, made fun of, and on one occasion shot at five times with a repeating shotgun at close range*!, so bear in mind that WE don't know what was on the guardsman's mind,or what he had recently been through!
That said, although I applaud what he did, it was still wrong (as per code of conduct) and could have better been dealt with by an outsider summoned from the sentry boxes foot buzzer (if fitted)
*As for the shotgun incident, my trusty pick axe handle (no weapons in those days) was applied surgically the the assaulter's head, and he decided to lie down and rest there and then.
