Mercury was unprepared for Hel's ambush and could not defend against it.
The Underground Shelter dungeon is located in the Middle Layer, not having entered the Realm of Reason, also subjected to the plane barrier's limitations.
The body of Mercury appearing in the secret research facility was merely a projection of a God, not his true self.
His projected body, boasting Divinity, was able to chase the Painter into fleeing in all directions, but when faced with Hel, who had complete mastery of Death Divinity, it was inadequate, and he could only be abused unilaterally by Hel.
He wanted to conserve his strength and escape with his projection, but within the embryonic form of Hel's Divine Kingdom, he couldn't even manage to leave.
After merely a few minutes of defense, he was decapitated, and the projection dispersed.
Before acquiring Divinity, Hel was just an ordinary Surface People girl and had no extensive knowledge about Gods.
