Well, a few things here. Firstly, in my own life, I don't really remember a situation where I've had to do this. I mean I've seen people getting into fights over stupid things and I've gotten in the way to stop them but that's about it.
Also, if you're referring to this specific case or cases like this where horrible things happen and people just let it happen; well, there are some factors you have to consider here.
The thing is people, as individuals, would usually be afraid to do anything because they'd wonder if anyone else would come to support their cause. Say, a guy is getting stabbed and there are 10 people watching. Something can be done if all 10 or a large number of people stop this (even in this case there's a chance that one of them might get stabbed). But in that moment, everyone would be wondering, "What if I go alone and stop this ? What would the repercussions be ?". So instead, they do an instinctive cost-benefit analysis in their head (or rather their head does it for them in the heat of the situation) and decide that it would be safer overall (from their point of view) to voice their outrage AFTER the incident takes place. It's like, better him getting stabbed and later all of us voicing our outrage than me getting stabbed and all of them voicing their outrage. As horrible as it sounds, it's the ugly truth. .