Civil law has a concept called the "
eggshell skull" doctrine, which basically says an attacker takes the victim as he finds him -- that when someone commits a negligent act that leads to another person getting injured, then that person is culpable for the
entire extent of the injury even if the victim had some pre existing condition that turned what should have been a minor problem into a major one.
In criminal law, a similar doctrine applies in self defense. The attacker doesn't have to know he is putting you in legitimate & reasonable fear for your life.
You simply have to know it and be able to articulate your reasonable belief that (for example) someone about to punch you in the head was actually about to use deadly force against you whether he fully realized it or not.
See
http://www.useofforce.us/ for a really good overview.
(Disclaimer: IANAL, simply an autodidact with a wickedly obsessive reading habit.)
pax