This very thing once happened to me and the results of me interacting were crazy to say the least. I saw a mother hitting her child at the grocery store and it went on until the child was on the ground crying and screaming. I couldn't keep from saying something, so I went over to the lady and said, "I think she gets the picture, you need to calm down." Well these were the famous last words....
The lady looked at me and transfered the hits from her daughter right to my nose. I leaned forward with blood coming out of my "now" broken nose and that's when she kicked me in the face. That's also the last thing I remember. The next thing I know, I come to and there's people all around me staring down at me asking me if I'm OK. I can remember replying, "NO!, No I'm not OK."
Turns out the person that beat the crap out of me was drunk and got pulled over by chance. A police car was passing by as she was tearing out of the parking lot right after she did the job on my face.
So, moral to the story?.... I think if you see something happening and disagree with it so much that you feel you need to do something about it.... go ahead and speak your peace of mind or get involved.... just be prepared to defend your life. People get killed for little these days so be aware at all times.