One might argue that Clary's response to the wife's 'blow'
(if it were, for example, a slap) was disproportionate and
beyond that required in self-defense. Therefore, the narrator
might have been justified in escalating the matter, what with
his wife on the floor like she was dead.

But perhaps not--- the narrator would have had to be in fear for
himself or another to use the force that he testifies to (i.e.,
slamming Clary' head into the parlor door, apparently repeatedly).

On the other hand, the final escalation comes from Clary, who
produces a knife.

No doubt the narrator would be tried for 2nd-degree manslaughter
and would make the self-defense argument.

It would then be up to the jury.