police applying street justice. He ran, they punished him by beating him.
This is about the worst crime it's possible to commit. Not because of the impact on the victim, serious though that is. But because it's a crime against the entire structure of society.
Police are not allowed to punish people. Courts punish. Police detain and investigate.
Appropriating to oneself powers reserved to the state, and abusing your position as an agent of the state in order to achieve that appropriation, is as bad as it gets.
Inflicting grievous harm on a person is a lesser crime than inflicting it on society.
Police who exceed their authority should be penalised proportionately to the magnitude of their crime - they should be looking at life in jail for even conspiring with colleagues to punish any citizen, much less actually meting out such punishment.
"street justice" is just another way to say "police overreach and brutality". Justice is the sole prerogative of a court of law.