Sorry, missed your question. As I see it; the cop does not want to take part in the violence, he has to because the criminal gives him no choice. Therefore he is protecting himself from violence he does not want to be a part of. So for me this is SD If you get into an argument in bar (as an example) and the guy offers to "step outside and settle this" if you agree you have consented to take part in the violence. You could have said no, you could have walked away, but you consented to the fight. This then makes it very difficulty to claim SD when you get to the Police Station. Obviously no one consents to being mugged or attacked, so protecting yourself from non consensual criminal violence is SD.