So the guy had a knife out, and he allegedly pulled it because because the cops were stopping him over skipping fare. Causal issues here are mental health issues, poverty, and the fact that the fare creates a tension that exacerbates the former two issues which is then brought to a head by the cop.

The cop not being there gets rid of this situation, as their existence in a social group implies and creates violence and getting rid of the fare stops this situation.

All the takes I see about this are ignoring what actually happened, which is annoying and doesn't really help understand the situation.