I think the reason is because the US has a "one fits all" approach to the police force, i live in Mexico, we have the cartels, sometimes the police are the cartels and we still have few people being killed by the police, so its not a matter of violent criminals.
The issue is that in most countries there are a division of cops, so the cop that is fighting violent criminals, terrorists and drug cartels are not the same cops that pull you over and write you a ticket.
So while most countries have a militarized police, in those countries these guys are not doing routine traffic stops turned drug search turned shootout. These guys are only called when there is a violent crime.
In the US you have a one cop fits all, so they deal with common people the same way they deal with gang members, that cant end well