Rulings

You can’t choose a creature that hasn’t yet dealt damage during the turn as the target of Executioner’s Swing, so in most cases it’s not possible for Executioner’s Swing to reduce the amount of combat damage a creature deals. (If a creature has double strike, you could cast Executioner’s Swing after it deals first-strike damage but before it deals regular combat damage.)
It doesn’t matter what the creature dealt damage to or if that damage was combat damage.