Mercy was a rare virtue that Desmond did not practice; he knew that to have mercy on your enemies was to hurt yourself. See Archibald on his knees, grieving the loss of his child; though tragic, he didn't move Desmond's heart in the slightest.
This was the price of their choices, which they found at the end of their chosen path. Desmond couldn't afford to be merciful to go further on his path.
A gentle breeze swirled around his ankles, quickly enveloping Desmond's legs in a faint whirlwind. With Desmond as the starting point, an almost indistinguishable wind tunnel was generated; Desmond vanished from one moment to the next.
Almost instantly, Desmond's figure materialized again, at the end of the wind tunnel, right behind Archibald. Behind him was only a trail of chaotic wind and a flash of silver. A moment later, Archibald's head was severed from the rest of his body, falling to the ground with a thud.