If a group of people dressed up as zombies and knocked on my door then pretended to get me when I opened it and I killed one of them. If I genuinely thaught they were zombies, would I have comitted a crime? Iv'e just watched a zombie film and was wondering.
self-defence means you have to be defending yourself. It's not enough to kill someone because you thought he was a zombie, you have to think you're in danger. You can't just rush out on Halloween shooting everyone. You could try the defence in court but the prosecution will say 'Are you really dim enough to think there are zombies at your door?' So you'd better make sure you look like the village idiot when you go to court.