I am sure we would all like to think of ourselves as people who would help someone in distress. One poor chap did today and died as a consequence. While I may have had a brother wrestle when I was a kid, I have never physically fought against anyone. I would not feel confident to physically tackle someone who I saw doing wrong. Not quite true. I have in the past but only after making a quick judgement of the person, and it was with words, not physicality. But it would seem that was all that this dead person did.
From what I know of today's incident, I would have left it well alone. There are times I have intervened and times when I haven't.
But if you don't, what do you do? Well the first thing is call the police if there is physical violence involved. Then observe and note. While you may not be the hero of the day, you could be the person who solves a crime by your observations.
In fact, for situations like the one today or just maybe something much more minor, if it is a breach of the law of a serious nature, call the cops. That is their job and theirs to prioritize as society dictates.
It was an awful thing that happened to someone going about their business today. He just tried to help a damsel in distress.