Somewhere along the way, from becoming a ninja to here, Kakashi has lost his heart. It must have crumbled away slowly, piece by piece in between losing itself in big chunks when his father was shunned and later committed suicide. Between when he first saw war and when he first killed. He can't pinpoint the exact moment where his humanity cooled to a stop, but he knows with complete certainty that his heart is gone now. Has been gone.
Just like Rin's, shuddering and twitching to a stop with the influx of electric chakra under his own fingertips and arm. Down to his elbow in her chest. He feels a visceral pain envelop his chest, mirroring hers, where he knows he should really be feeling shock.
He faces every nightmare he has ever had and his mind is breaking. There's something psychosomatic occurring, and he almost thinks it's Obito's eye. Obito, getting back from the dead to punish him for breaking his promise. He said he'd protect Rin, and now she's dead, and he caused it. Of course Obito could manage to pull something off like that; his emotions were always so bright and strong. Strong enough to reach past the barriers between the worlds.
In some bittersweet way, he's glad that his teammates are together now, in the Pure Lands. Obito saw him kill Rin, so he obviously will reunite with her now. They can live forever in perfect happiness and hate the person that killed them both. They should. Kakashi deserves every ounce of contempt they have to give for him. He doesn't know how he will manage the weight, but he can find out.
With Rin gone, his promise betrayed, his comrades betrayed and himself marked as scum, he doesn't know what to do. Continue the mission? Kill the Kiri shinobi? He has to protect Rin's body, and make sure Obito's gift is safe. Yes. He must. He knows seals for the destruction of his body and its secrets, but he doesn't know if he is worth it. At the back of his mind, something tells him there is a better solution. Something more meaningful.
He wants Obito's eye to be safe, but also out of anyone else's hands. Obito's clan never wanted him, so what do they have to do? He could seal it forever, use it for one last piece of good and destroy it, or perhaps try out the new clarity of Obito's eye. He pauses, slightly, from his rage at the thought of this being the last Obito sees. He was supposed to show Obito the future. If there isn't any future for this eye past now, Kakashi has at least a duty to not end it on this.
Obito's eye's vision blurs, a little bit, and he starts to push away what that means before he realizes there's no point lying to himself this late in the game. One half of him is crying. But the other half, the one that lost its heart way back when, keeps moving until he doesn't have to anymore.
Kakashi decides to kill all the birds with one stone. If there's anything at all he's good at in life, for better or for worse, it's efficiency.
He removes Rin's body from the area, and moves to an adjacent clearing in the trees. There isn't much beauty he can find in this gray world right now, but he tries. He looks up and sees the sun. He looks down and sees delicate flowers, shining in their yellows and pinks and purples under the sky, staining more with the Rin's blood as the seconds pass. His chest keeps hurting like hell.
With a speck of Doton, carefully controlled for his deathly lacking chakra reserves, he creates a coffin of earth for Rin. He lays Sensei's kunai in the ground above it, obtained from the pouch that holds his two most meaningful weapons. He breathes harder. Steps become labored, so he finally kneels.
He stares at the flowers and the sun and the sadness of the trees. He gives a silent prayer to Obito, and whispers an apology that could never make up for everything. Really, it only makes up for what he plans to do next.
He reaches with the hand that did not kill Rin and he applies a seal over Obito's gift. The eye should not be transplantable and functional in any other person, now. It does not destroy the eye, because he could never shirk that final and only gift. Poetically, the seal would allow only Obito to reclaim the eye. Maybe in the Pure Lands, Kakashi thinks, Obito will have both. Kakashi can make Obito whole again in the only way he knows how after destroying his life and entire world.
Rin always liked poetry. He wonders what she would think of this, poetic as he's desperately hoping he can make it in his limited ability and thinning clarity. Probably nothing, because she's dead.
Kakashi chokes himself back to seriousness. He draws from his pouch his other most sentimental weapon. It is a beautiful blade, built to channel their—his—their clan's cutting white chakra. The construction is high-quality, and the colors mixing blue with bone with gray please him somewhat. His heart isn't real so it does not cease to beat now. His mind bleeds all color but the grim.
Somewhere, dimly, almost beyond his perception, he hears the falling of a body and a sudden lack of noise from the clearing where he killed Rin. He almost wonders if the Kiri-nin are all dead, or if they've been attacked and killed. Then, he knows not to wonder.
Kakashi thinks of poetry, and a girl. He thinks of wanting to have a heart, and failing where he was needed most. He thinks of Obito, bright and kind and sacrificing and asking just one thing back after an infinite amount he gave Kakashi. A gift, an order of protection. Broken promises. Red eyes. Red blood, cooling on his hand; on the floor of his family home. He thinks of repentance the only way he knows how.
He draws the blade across his lower torso. He pushes it deep, left to right. The resistance he feels indicates the blade scraped his spine. He places the blade calmly on the ground beside him, perfectly orderly with his kneeled legs and neat hands. He feels oddly calm. His mind quiets its screaming of grief and guilt, just slightly.
Kakashi closes his last eye, because seeing the flowers is only for good people like Obito, not scum like him.
Kakashi dies.
(Obito arrives to see it happen. After a blind rage of killing the Kiri-nin, he realized he could no longer see her body. Nor Kakashi, for that matter. He somehow bubbles up with renewed rage, orange and acidic, and he follows the direction he knows Kakashi stole her to.
When he arrives, Kakashi has one eye closed, *his* eye. There is a tanto, the one that killed his father, exiting out of his body from the right side. He watches Kakashi close his remaining eye. Obito falters. Obito freezes. He stares at Kakashi's face, lined with pain and acceptance and some small measure of satisfaction, and one part of his anger boils away. Acute nausea replaces it.
He wanted Kakashi to suffer for killing her. Just, not like this. Never like this.
He is still for a long, long time.)