Wrong is wrong

This morning, we were on our way to the park for a run. We were behind an elderly couple - the man was driving & the woman was the passenger. No one else was visible in their car. The woman punched the man in the eye, while he was driving. She appeared to be ranting at him. She kept hitting him periodically in the face, slapping him, while he was driving. He never raised his hand to her, at least not that we could see. We decided to follow them in case the man needed help.

We followed them to a public library, where the woman hit him again when the car was stopped. He exited the car & was so downtrodden it broke my heart. I confronted her while Tony called the police. I don't remember exactly what I said to either of them; Tony tells me that I kept telling her she couldn't hit him... she wasn't allowed to hit him... it wasn't right for her to hit him. I remember her telling me he was her husband & she'd hit him if she wanted, that he was a stupid retard. I wanted to throttle her. When she said she was going to hit me too for being nosy, I wished for it: I wouldn't have hit back, but I would've pressed charges against her. She kept telling me it was none of our business, and I insisted that wrong was wrong which made it my business as a good Samaritan.

Eventually the police came & dealt with her; the man kept calling me Miss and asking us to leave it alone, to stay out of it. It was horrible. I still feel so badly for that man. If I had to guess, I'd say the man had been mistreated for so long that he couldn't see anything else. He had the demeanor of a cowering whipped dog. I hope he doesn't live like that every day, but even once in awhile is too much.

I'm considering the possibility that she could have some kind of illness that made her act that way, but she was so lucid & so mean that I think it was just her personality. She radiated anger & hatred, if that makes sense.

How do you know when it's right to get involved? Isn't being nosy just a matter of perception? I mean, if we had come across that same couple in a different circumstance - say, they were broken down on the side of the road & we lent assistance - they wouldn't perceive that as being nosy, right? They'd probably perceive that as being helpful. And the woman saw it as nosiness today because she didn't welcome the help we were trying to give to the man.

What's right when things are so messy? Aren't we supposed to stand up for others when they're being mistreated?