Not holding policemen accountable when they kill someone has three effects:

1. Other policemen will hesitate less before killing someone.

2. Citizen will be more afraid and protest less in front of policemen. (the more the news speak about it, the more afraid they will be)

3. The few remaining protestors will be more violent and thus killed more easily with less testimony.

=> Goto 1.

There, you have it: the purpose of a system is its effect.