What would you do if you saw someone beating their dog as if it was brought home by the red-headed step child?

If you saw a person beating their dog because it did something (you do not know what) that caused its master to be injured, the animal was yelping but could not escape for the collar and chain, what would be your reaction? Would the size of the person doing the beating have any sway? Would it be determined by the size of the dog or the breed? Would the object which the person was using to administer the beating with make a difference? If you were armed would you pull your piece on the owner to make them stop if yelling at them to do so did not work? Would you shoot them if armed to make them cease beating the dog?