i can tell you from experiance, most dogs do not know what it means when a gun is pointed at them. but many (and this saved my butt one day) do know that guns sound scary when they go boom.
this is what can be threating to a dog: eye contact, posture, hand gestures above their heads and moving down, vocal threats by tone (growl, yell), and getting in their face. which is why, if you must approch an aggressive dog, you face away from it, you do not look it in the eye, keep your movements slow, speak in a nonthreating soft tone, and slowly back towards it (but make sure you have room to get away, if it don't work).
its also why you do not stick your face in a strange dogs face and you keep your hand under its chin, palm up and let the dog make first contact.
if the dogs were threating the guy, i've got no problem with what he did. dogs, especially if you have multiples, running around lose can be very dangerous. on the other hand, if the dog wern't being a nusance and the guy was just being a jerk, he should get some trouble from the law (not sure how the animal crultey bits gonna work though...)