This is a thorny and ambiguous question - and one that assumes that killing someone is the only form of self-defence.
In most situations of self defence - the person defending themselves will have a number of options open to them other than killing their adversary.
In most cases - if death is the apparently "accidental" or "unintended" result of the situation, then the charge would be manslaughter - or accidental manslaughter.
i.e. - if you hit someone on the head with a heavy object to protect yourself, and they later died of that injury.
Realistically it's a very difficult thing to argue in court. If you had wrestled a gun off an intruder, you would not have the right to shoot them (not in the UK anyway). If you were to shoot them having gained the weapon, this would be considered an act of anger, retribution, and premeditation - no matter how brief the premeditation.
Realistically self-defence is a term that refers to your instinctive reflex behaviour when attacked. Anything outside this is a response rather than a reaction.