Sharingan
I think all Sharingan users must've been cursed (just look at what happened to Hanabi's child), but I wonder if Kakashi knew that before accepting his eye.
It was told Hatake Kakashi could mimic anything using his Sharingan, hence his title the Copy Ninja.
I never knew 'bout it directly, but Iruka whispered to me about Kakashi's Sharingan when he was gone. He leaned over and whispered, "That eye tore him apart, you know."
I didn't understand.
Apparently, jutsus weren't the only things his eye memorized. People. Places.
Deaths.
Kakashi's eye remembered every enemy, comrade, and friend it saw fall, and it replayed those memories for him, mockingly, when he laid down to sleep at night.
Iruka said Kakashi used to talk in his sleep.
I said, "Kakashi talked in the day, too. So what?"
Iruka only shook his head sadly and said that wasn't it. Kakashi used to talk to himself, to his eye.
I vaguely remember Kakashi's lips moving silently beneath his mask.
I guess he wasn't chewing gum. So much for Sakura's theory.
I guess it makes sense, though.
Kakashi was famous among the Konoha spooks, also known as ANBU, but he had his own spooks, too. The ghosts of every ended life he witnessed.
According to the few ANBU members still around who worked with him, he talked to his ghosts all the time. His eccentricities (a nicer term for his insanities) only bolstered his reputation, making him someone even ANBU crossed to the other side of the street when passing.
Out of respect for their captain, his ANBU subordinates looked the other way when he simpered to himself (and covered their ears when he shouted), so they never knew exactly what he said to that eye.
He was never apologizing.
Only pleading:
"I saw, I see...
"Just leave me be!"
On solo missions, doing god knows what for ANBU, he probably had full-blown fucking conversations with that eye.
However the conversations went, they weren't pleasant; he always stumbled back, the skin around his eye torn and bleeding, ever mouthing, "I saw, I see," over and over, "Just leave me be!"
After he got lost and collapsed a half-day's trek from the Konoha gates, Tsunade made sure he never went solo again.
Not more than two missions later, he was being half-carried back to the village, blinded by his bleeding eye, accompanying ANBU trying to ignore his captain's muttering.
For a long while this tactic work; send Kakashi out with some new ANBU schmuck, have him to the dirty work, and the new guy leads him back. It saved Kakashi's ass a fair few times, and it was good exposure for the new member.
A good number of ANBU rookies quit after their first mission with Kakashi.
After about a month or two, though, Kakashi's partner was killed on a mission to assassinate the remnants of the Kamizuru clan, a request from the dying Aburame Shibi without the knowledge of his clan.
His partner was dead, but Kakashi worked through the rest of the clan, killing every aged man and young child, his eye recording it all, and replaying it, overtime.
Kakashi did the only thing his broken mind could see fit: he clawed his eye out, shouting:
"I saw, I see!
"Just leave me be!"
The rush of deaths, the reminders, they must've stopped, because he sat down, holding his bleeding socket, in the middle of a dirt road. He didn't have to watch Obito offering him the eye with his dying breath, he didn't have to watch his own hands kill innocent civilians for ANBU, he didn't have to watch his friends and coworkers fall.
He laid down in the dirt, waiting to be carried back, because that was how it always worked.
Without the eye, he couldn't remember that his partner was dead.
We never saw his body. ANBU took care of it.