At what point would you step in?

Right up in town here there was a guy who during an altercation with his wife or gf or whatever he starts to beat her up in their van. I've seen guys grab their Wife/GF a few times. Typically you see the impatient parent who has a screaming child and they get overwhelmed and tend to get a little to rough on their kids. Now having a son I do understand getting frustrated, but there is a fine line between to rough and just frustrated.



Have you ever had to step into a situation for someone who couldn't defend themselves? If so or if not, at what point do you feel it's necessary?