Only by personally experiencing great power and despair can one truly remember the feeling and strive to never fall into despair again.
Placido was the Viscount of Watchtower Castle. Given Ronin's power and the vast disparity between their forces, his choice to surrender was understandable.
But from Ronin's perspective, if Placido were to lead troops into battle in the future, would he just surrender again when faced with a powerful enemy?
Ronin valued his status and strength as a Meteor Knight, as well as Melis's identity as a Mage.
But as a Lord, he had no desire to see his subordinates betray him.
He hated fence-sitters.
So, he needed to make Placido witness his power and personally feel the insurmountable gap between them. This would make him think twice before ever entertaining thoughts of betrayal in the future.
On the city wall, Placido's expression grew even more grim when he saw his Sixth-level Magic Cannon's blast dissipated by a single slash of Ridder's sword.
