Could be considered spoilers for the secrets of the Uchiha. But, it was all conjecture before the fact, so take what you will from it.
Itachi killed with a quick efficiency. It wasn't anything new, killing off a clan. He'd done it before on missions far from Konoha, being a good ninja and listening to orders.
But this was his clan. He stared into the eyes of the woman that pulled him from his mother's womb as he plunged his sword into her gut. He swung around and stabbed her husband through the throat -- he had given him his first kunai. The streets were filled with bodies of people that had been there when Itachi took his first steps, lost his first tooth, turned three, and then four, and then five.
And he cut through them with a cruel efficiency that betrayed his hatred of them.
He left his own residence for last. It wasn't to relish killing his parents; he despised their ways and what they planned, but he did not despise them. His mother had nurtured him for nearly 10 months in the womb, she had named him, raised him to be a fine boy and ninja.
She had also put the target on her own back when she married his father. When she decided to remain with the clan, she had signed her own death warrant.
And his father had been the one to hand it to Itachi.
How stupid could they be, planning to attack Konoha? They had worked so hard to make Itachi the perfect ninja, yet they thought his blood would overturn the oath he made to the Hokage? Itachi did not relish killing off his clan, but he knew it was deserved.
The front door banged open.
For this, Itachi was sorry. He was going to crush Sasuke's entire world. The boy wouldn't understand that it had to be done, wouldn't understand that Itachi did it for Konoha, but most of all, for Sasuke himself. Danzo demanded the boy's death too; he probably planned to have Itachi assassinated on his way from the compound, claiming it was done to weed a deranged killer from the village's midst. He wasn't sure what the elders had planned -- Homura was unpredictable and Koharu demanded blood for the smallest of disloyalties, though Itachi had yet to do anything he considered disloyal. Not to the village.
By Itachi taking this mission, he had saved Sasuke's life, though no one knew that yet. He had an idea that the Hokage had figured it out -- he wasn't nicknamed the Professor for nothing -- but he hadn't said anything, which meant his plan would work.
Sasuke would hate him, but he'd live. As long as Itachi betrayed the village.
It ached, thinking about that. The jounin gave himself a moment to feel that pain as he waited for his brother. He was a good ninja, loyal to his village and his kage. If it hadn't been for his little brother, he would have just let them kill him.
But then Sasuke would die. By betraying the Hokage, the elders, Danzo, and every principle he had ever lived by, he could save his little brother; by blackmailing the village he loved so dearly, he could protect the only thing he loved more.
Sasuke burst into the room. He didn't see Itachi. All he saw was the puddle of red in the middle of the room and the bodies of their parents.
Itachi closed his eyes for a long moment, something no ninja ever did, something no Uchiha ever did.
Then he stepped into the moon light.
Everything went by far to slowly after that. As he tortured his brother for what felt like an eternity to both of them but was only a few seconds in reality, he knew this was the end. After this, he could never return to the Uchiha compound or to Konoha village. He could never be Sasuke's big brother again. He would be Itachi Uchiha, missing nin of Konoha, murderer of the Uchiha clan.
And Sasuke would hate him.
That needed to happen too. Sasuke needed to hate him, to grow stronger so that someday, maybe he could kill the legendary Itachi. He needed to be able to hold off Madara and the Akatsuki. He needed to be strong enough to hold off Danzo and the elders once he did kill Itachi. And he would. He would fight Itachi one day and kill him. Itachi couldn't die before then, he wouldn't let himself.
For all of this, Itachi didn't regret a thing. He would never be able to return to Konoha, never be able to return to the only home he'd ever known. But that wasn't what he considered home anymore.
As long as Sasuke lived, he'd have a home in this world. It didn't matter that he couldn't be with him, all Itachi needed was the knowledge that that home existed.
That was enough to keep him alive.