Never, far too many shades of grey and you can get the decision wrong and murder someone incorrectly far too easily.
I know where you are coming from, in cases like the one where that Michael Adebolajo chopped that Lee Rigby's lid off in broad day light in the middle of the street and was shouting on about Jihad with blood all over his hands and admitted it and video footage, witnesses etc there is no case for error and you could argue it therefore has a case, however where do you draw the line?
Life will throw up cases where evidence becomes questionable and that is where errors can be made, life ain't black and white, plus it's fucking barbaric, two wrongs don't make a right I was always told.
On the doorstep [Siouxsie] asks me whether right or left would be the best direction to find a taxi, and although her best bet would be left, I suggest she turns right. It is churlish of me, but it is she who has set the pace.