So what happens if Rain accidentally does kill?

So we know that Rain can attack Nightmare Creatures if she is absolutely certain that she wont kill them by hitting.

What happens though, if for example, she shoots the arrow, in the meantime the Nightmare Creature gets hit so gruesomely, that its literally one second before death and the arrow hits and attributes the kill to Rain?

Does she get punished for it?

Will she just... implode?

What will happen?

Or is it something like, if by any chance the spell knows she will kill the enemy even if she herself doesnt think so, then it still will prevent her from attacking?