Would killing a knife attacker at the scene be considered self-defense?

I'm curious because I heard that self-defense is rarely recognized in Korea.
If I tried to subdue a violent knife attacker at the scene by swinging a baseball bat,
and the attacker died from a blow to the back of the head, would that not be considered self-defense?
I saw on YouTube that in the US, when an armed robber enters a restaurant, customers shoot them without hesitation;;;
But I heard they don't get punished in the US either.