The outrage to this is honestly due to the "shocking the public consciousness". This is not something the public thinks about happening, so it is "shocking" to see something like this.
I asked before and didn't get an answer from anyone. What if the cop had only hit her one or two times? What if it was a man rather than a woman in the same circumstances? What if the genders were switched completely? What if it was a civilian (of either gender) who was trying to "save" the woman, who then started hitting the civilian "helper" and that civilian did the same thing to the woman? Would the reaction from the public have been so bad if the same thing had resulted, no actual injury to the person?