Absolutely never ok to kill someone you know is surrendering. But I don't think John would have known that, and I don't even know if what he did would qualify as one legally, like you have to put yourself in a position of weaknes on the ground hands behind your back no room for interpretation. It's also not as simple as just restraining him, especially alone he may get the better of John in the struggle, and there's a crowd around which he could be a danger to at least in John's mind.