Would you hit back?

Fight or flight. Two very basic instincts embedded into our behavioural fabrics.
In Singapore, however, another instinct we have is to camouflage.

During lunch, my friends and I had an interesting debate. We were engaging in P2P experience sharing when one of us recounted how a lady screamed that she was molested on the train. She very violently 'apprehended' the culprit. Obviously, she needed no help making herself heard. but as usual, most bystanders simply ignored her calls and made it none of their business.

I form part of the majority. I don't really care whether she was molested or not.
Instead, I posed a perplexing dilemna:

If you are guy,
and you stumble upon a couple caught in a heated argument. Suddenly, the man slaps the woman.


What would you do? What should you do?
Should you take a firm moral stand, step in and tell the man off?
Should you step in to just break up the fight and prevent it from escalating?
Should you stay back and observe from afar, only to step in if the situation escalates?
Should you just STFU and stay out of other people's affairs?

At the end of the day, it really boils down to how concrete you see your abstraction of moral values are. There is a very fine line between mutual repsect for privacy and a coward. There is an even thinner line between courage and being a self-rigtheous brat.

My friends, what would you do?