Why are cops allowed to do what they want?

Some pigs in porto alegre killed a 16 year old who was apparantely un armed after they got into an argument with the teen. I used to want to be a cop but this is like the 1000th time I've heard of something like this, and quite frankly I'm so disgusted with the behavior of some cops I d ont want anything to do with them. They would adjust their behavior if they faced reprocussions for their childish behavior, but instead they are pardoned and go on to lie and do more damage to the ppl they are sworn to protect. Why do they have a license to kill? P.S. If your offended by my post your the type of coward I'm talking about.