A friend of mine was held up by 2 guys an underpass, 5:40 pm on a weekday. He looks like your typical out of condition middle aged bloke, perhaps a bit fat. Sadly for his would be muggers he's actually 6 foot tall and he looks fat because of all the muscle he carries (ex professional American Football player). Mugger pulls knife on him, red mist comes down and one mugger is unconscious on the floor, the other one runs off. My mate has a slashed jacket but no injuries (except knuckles).
My mate then ended up on a 36 hour bender because he thought he might have killed the guy. Next thing I see of him he's trying to resign from work – we wouldn't accept his resignation, it had nothing to do with work and was all hearsay.
BUT what if he had killed the mugger? I'd hope that it would have been seen as self defence but he would have had all sorts of trouble proving it.
Not saying don't do it, just be careful out there.