He wanted to claim an affirmative defense that he shot in order to defend others, since the cop who was shot had drawn and was aiming a firearm. The judge prohibited the defendant from bringing that defense claim (not that it likely would have worked).

Frankly I don't see how a cop presenting unwarranted deadly force is different from a random person doing the same. Especially now that we've had a decade of body worn camera footage to prove just how lawless American police are.