I love how the manga takes the OP power of instant killing to its logical extreme. A lot of problems are bypassed easily in a place where killing is a perfectly viable solution. It's pretty fun to see how OP this can get.
"you defeated me, but i'm just the weakest of demon lord's four heavenly kings! hahahaha---"
"uh, sorry, i think i already killed the demon lord... by accident"
"--haha....wat?"
Fucking hell, MC. Though, why didn't the effect worn off since there's no one else alive? Or is it because he only killed the Demon King but the Holy King is still alive?