Reisi thinks the smile will be the worst of it.
He knows what he has to do. He has been bracing himself for this for days, gaining mental traction on the idea through repetition as it has become more and more clear that nothing he can do will sway Mikoto. And he has his duty, his responsibility to protect his own people and Mikoto's people and everyone, insofar as he can, and in the end he has always held responsibility higher than his personal feelings.
It's still harder than he expects. He doesn't hesitate - by the time the moment of truth comes any hesitation would be suicide, worse, would be the murder of dozens of innocents, and his trained instincts don't falter, his body does exactly what he has been telling it to do. But as he comes close enough, as he draws back the shining edge of his sword to thrust it through Mikoto's chest, he sees the other man's expression.
Mikoto's smiling, his arms hanging limp at his sides in clear submission to exactly what Reisi intends, what Mikoto has intended for them all along. The smile is bad enough, the lopsided curve of the mouth Reisi can remember the taste of, but the warmth of it is bleeding up into his eyes, turning his expression relaxed and calm with the loss of tension Reisi didn't even realize was there.
His hand still follows through on the motion. Muscle memory is stronger than the sudden shove of emotion that closes off his throat, that tells him to stop when it's far too late to catch back the action. Mikoto's body doesn't resist any more than Reisi's own trained muscles did; the blade slides straight through him, tearing through cloth and skin and blood alike so Reisi stumbles forward, nearly falls as his sword settles into Mikoto like it was meant to end there.
Reisi can't move. His head is echoing with the first shock of horror; the best his imagination could muster is useless in the face of the reality, of Mikoto smiling at him like he's been given a gift while his blood spills red over Reisi's hand and Reisi's uniform. There's no way to avoid the reality of it, and in the first burst of desperate denial Reisi thinks he might pass out as all his skin flashes icy with shivering refusal of understanding.
Then Mikoto's hand lands heavy on his shoulder, and Reisi blinks, and he can't deny it any more.
"Reisi." Mikoto's voice is soft, so soft, even the usual low rumble of his voice has gone quiet with pain or peace or both. He coughs, shuts his eyes for a moment, and Reisi's horrified gaze catches on the trickle of blood that spills past his lips to paint his skin as red as his hair. The hand at his shoulder slides sideways, heavy like Mikoto can't even lift the weight of his own hand, comes up across Reisi's coat until the other man can angle two fingers up to press against the bare skin at the top of Reisi's collar.
"Thank you." There's no sarcasm there, no irony or bitterness at all; if anything there's an apology, gratitude so strong Reisi is certain he will drown in it before he can remember how to breathe, before he can fathom how to exist at all with Mikoto's blood on his hands. Mikoto leans forward, and for a cold moment Reisi is sure he's collapsing into the other man's arms, that his failing strength can't hold him upright. But it's not quite as boneless as that, there's still a deliberation under Mikoto's movements, and Reisi realizes what he intends a moment before Mikoto's mouth lands on his.
It's not fair that he should still be so warm. Reisi's sword is through his chest, Reisi's clothes are soaked in his blood, and Reisi is still the one shivering while Mikoto is smiling, Mikoto is sighing heat into his mouth. Reisi opens his mouth against Mikoto's lips to try to catch a breath he can't use, and his tongue goes metallic with Mikoto's blood. His heart stutters back into overdrive, desperate with the immediate threat of loss, and his free hand comes up to clutch at the back of Mikoto's head, hold him still and present and close for another heartbeat, two, three.
The fingers against Reisi's skin go slack. Reisi makes a sound into Mikoto's mouth, a whimpering sob desperate for more, for a minute or a second or a breath, and the tension that drags Mikoto's lips into the edge of a smile tastes like an apology. Then his arm falls away, and all Reisi's efforts to hold him in place aren't enough to hide the weight that drags Mikoto impossibly heavy and away from him.
The salt of tears touches against the bitter of blood on Reisi's lips, but he doesn't taste anything but ice.