G20 demonstrations

If you annoy me and i then take it upon myself to punch you in the face then i am the one that is wholly responsible for the fact that I punched you in the face, as i didn't act proprtionatly, and it becomes even more important when I am a police officer

No, you're wrong. If you annoy me, used abusive language towards me, mock-punch me, and then I strike you, then that is different to me striking you for no real reason. Any court, in any country, will take your actions into account. This will undoubtedly affect my punishment in striking you. You will have to accept some responsibility.

And the police have do have a much greater ability to use physical force on someone than an ordinary civilian, whether you like it or not.