well suppose the devil's only there so we have an opposite to good
The pain is not instant, nor does it force him into shock as he was sure it would. Instead, the world becomes crystal clear around him. He watches Arthur pull back, heartbroken and steady, before removing himself completely from Mordred and Mordred falls without anything to help him stand.
It doesn't hurt, not really. It's no more pain than the aches and bruises he received during training all those days ago that feel like years. In the hazy air that spreads across the battlefield (or maybe it's his eyesight going) he can see a figure approach, right as Arthur falls to his knees. Mordred can hear the non-existant screaming in his head and he knows, suddenly, of what he's done, of the things he's ruined.
Merlin appears, beautiful Emrys shining bright white through the red haze. Morgana had been a deep, thick green, a kind of green that was reminiscent of thick sludge, over powering and instant. But Merlin is a gold so bright he turns white and pure
Oh, Mordred thinks, because he understands
The sword Morgana's given him falls limp from his hands. So that's why you hated me, he thinks, and if he weren't dying he'd find the energy to apologize, to thank Merlin for all those years ago when he could have let him die, but he didn't. Merlin proved, once again, that he was good, just like the stories said.
It's not so bad being the villan, he thinks when the red haze begins to turn black. After all, without a villan there could be no hero, and what kind of destiny is that?