I live in a country where we don't inject people with
   ketamine on the street, nor do "criminals run amok". 
   We also don't have armed gangs imposing "their own idea 
   of a justice system", and we have fewer police killings 
   in 25 years than the US has in 25 days.

As an American, the parent poster is an unfortunate and common example of the sort of attitude that is common here. It is at least partially the result of America's geographic isolation: most Americans have never been to a place where there is low crime and low police brutality and thus literally cannot even imagine it.

I would like to point out the America, one's treatment by police varies greatly.

I'm a white man in my 40s have never witnessed police brutality firsthand. In all my interactions with police, they have acted properly and de-escalated things nicely, even when perhaps I didn't agree with them.

This is the experience shared by many (most?) Americans who, once again, have trouble imagining experiences other than their own. They imagine that nearly all police are just, and therefore victims of police brutality must have done something to deserve the violence.

Essentially, it is the "just world" fallacy writ large. People like to think that the world is just, and therefore like to think that those who experience misfortune have done something to deserve it. It is more pleasant than realizing we live in a country where the police sometimes act in shockingly violent and inappropriate ways.

I do know many people who have experienced awful treatment by police. And of course in recent years much light has been shone upon this misconduct.