I have a supplementary to the question in my OP. How does the law react if someone highly trained in martial arts reacts, while unarmed, to being attacked (let's say with a knife, to even things out a little) with what turns out to be lethal force? I've never been involved with the law or law enforcement much, so I really don't know.
Sample scenario; said martial artist is attacked by someone with a fair-sized knife, dodges the initial attack and reacts with a throat strike.
Applies to the UK BTW.
The martial artist is perfectly within their right to deliver incapacitating force if their life is directly at risk in this scenario. That the attacker happened to die during the delivery of said incapacitating force is not an issue.
If the martial artist disarmed the attacker and
then killed them (without waiting to see if the attacker still intended to come at them), that's a no-no. The balance of threat and force is no longer justifiable.
I cannot think of a single martial art that
only has killing blows. A martial artist always has the option to
attempt to incapacitate their opponent, whether through dislocation or breaking of limbs, gaining enough distance to escape, or some other means. If the attempts fail and the attacker is still coming, the martial artist may have to resort to more fatal force - if, for example, their attacker is hopped up on PCP, they pretty much have no pain threshold whatsoever and a broken leg will not stop them (though you really should be able to outrun a guy once you've broken both his legs).
It's also always possible that you knock someone down, but in an unintended or unforeseen circumstance their head collides with the pavement, and
that kills them. This is also not the martial artist's fault, and our theoretical martial artist remains on the right side of the law.
This isn't to say there won't be any court case, but the majority of these things will get dropped before they ever reach a courtroom. If they
do get to court it's because the situation isn't so clear-cut.