Wanna fight?

I just went grocery shopping at my local supermarket and there was this pretty odd looking couple in front of me at the check-out. Actually, she wasn't odd looking, she was just normal looking, but he was this mean looking, really tall, fairly big butch guy, maybe about fifty. He had long wavy hair and a weird cap, baggy tracksuit pants and a leather jacket and looked kind of scary. I got this strange vibe from him straight away, but was telling myself not to be so judgemental. Maybe he was just a scary looking guy, but had a good heart. You never know.

Before I could finish thinking that thought, a fight breaks out. The woman just asked a normal question, something like, "So should I just pay this part by cash then and you put the rest on your card?", which apparently was too much for him. He started screaming at her, something along the lines of, "What the fuck! I fucking told you how we do it, we don't do it like that, we fucking do it like this!" He then proceeded to push her to the side, not much, but enough that she bumped into the cash register behind her. Then he loaded the groceries onto the conveyor belt, throwing each item on there like a maniac, scaring the check-out lady and a few people around.

His vibe made me almost physically sick and I was so close to saying something, but in the end decided not to. I'm usually the kind of person who will say something, but he really was super scary and while I didn't think he would have hit me, I had a feeling that if I'd said something, he might get even angrier and later take it out on the woman. But then I couldn't stop thinking about it, wondering if I'd done the right thing. I think in any case, if there is actual violence involved, say, if he had hit her, then people around have a responsibility to not just watch and do nothing. But sometimes words can be just as violent. And it does make me wonder why a woman would stay with a guy like that. What would you have done?