Perhaps you could improve my understanding by commenting on this story.
Fred is a night club bouncer. He is being prosecuted for assault as a result of trying to break up a dangerous disturbance. There is video evidence that shows that he struck Barney causing injury. At what point is Fred allowed to say that he hit Barney to prevent Wilma being strangled? Would his attempt to prevent greater harm be part of his defence (common law in this case?) or must he wait until convicted before offering an explanation as mitigation?
You clearly know better than me.
I have explained what a defence is, I have also explained what mitigation is.