The tricky part here is that you don't know if the security guard and the guy pounding him are old enemies and it has nothing to do with personal safety. The security guard might have been a hard-nosed prick who was egging the man on. Buuuuuuut....I see where you're going and we'll say that the man who is beating the security guard is more than likely up to no good.
I guarantee that if I pulled my piece and yelled some 'choice' words at him he'd stop. I've even heard of people pulling their pieces and shouting, "Stop! Police!" I imagine that would do the trick. Buuuuuut.....Lets say thats not the case and the man is handing out paddles.....Like that guy who got beat at a fast food restuarant in line and no one did anything....
Then, if I were sure that the man being beaten was in serious peril (life or serious injury) I would pistol whip the beat-er on the head or a boot to the face (assuming hes on the ground), which ever came more natural. Then I would at pistol point, hold the man there until police arrived.
WHEW!!