1. Before He Fights Itachi
Sasuke lies in wait, his muscles relaxed as he keenly watches a lounging Itachi from his hiding spot. Sasuke can feel himself approaching and his heart immediately starts to pick up as his chance to change fate draws closer.
He hadn't expected to be swept into the past. After sending off a letter to Naruto, a simple update on his location so his friend wouldn't worry, Sasuke had resumed his leisurely walk throughout the misty countryside in the Land of Water, the chilly mist dampening his cloak, leaving a welcome feeling against his naturally hot skin. He had been glancing up at the cloudy sky, trying to see if the morning sun would pierce through the thick gray clouds above him, when a sharp pain burst violently out of his right eye. His sharingan was siphoning his chakra at an alarming rate, the warm wetness of fresh blood tracking its way down Sasuke's cheek. He shouted in alarm, far too many sensations bombarding him at once, before his sharingan gave one final tug and Sasuke was no longer standing on a dirt road.
He had stumbled as he had clumsily found his footing, his hand flying up to cradle his aching eye. He kept it closed, jerking his head around so his hair would keep out of his other eye. He could feel his rinnegan pulsing in his socket, though it didn't hurt nearly as much as his sharingan did. It was when he finally let out a stuttering breath, his lungs releasing from their frightened clench, that Sasuke had stood up straight and actually thought about what the hell just happened.
He gently kept his hand pressed over his eyes, roving his uncovered one around his surroundings. He recognized it immediately, how could he not. He still has nightmares of Itachi's last moments, that frightening realization that he still cared about his brother. Most of his hangups were resolved when Itachi was reanimated for a short time, but that doesn't mean that guilt and shame don't still follow him around. Sasuke has thought about Itachi and his actions for what seems like his entire life. From betrayal and vengeance to outrage and understanding, finally landing on the complicated emotion of regret. Sasuke regretted his actions towards Itachi. He lived with that feeling every day.
Seeing the hidden Uchiha hideout standing without a blemish in sight drags his regrets starkly into focus. He suspects a genjutsu at first, extending his senses and tenderly poking at his eye. His sharingan has finally deactivated, leaving a watery dark eye in its wake. He blinks for a moment, trying to clear his vision, before he comes to the conclusion that he isn't in some genjutsu. Which means that his surroundings are real.
"Jikan," he mutters out, his eyes narrowing in concentration. It had been so long ago that his mother had explained the mystery that was the Jikan Phenomenon. The sharingan still held powers that even the Uchiha knew next to nothing about. Random Uchiha traveling throughout history was not unheard of in his clan. It was kept incredibly quiet, of course. No need to involve those outside the clan. The traveling Uchiha could either change the past and stay there to deal with the consequences, or not interfere and eventually be swept away by their sharingan once more, back to their original time.
Sasuke stared at the building before him and he knew, immediately, that he was more than willing to deal with the consequences.
So now he stands watching his brother, picking up on the signs of his illness that he had not noticed the first time he fought Itachi. His breathing is slightly labored and if he listened very closely in the silent room, he could hear a faint rattling in Itachi's chest. Sasuke can't imagine how much physical pain his brother is suffering through. A flicker of dark chakra spikes as a glaring Sasuke appears before Itachi. Sasuke doesn't listen to what they're saying, instead staring intently at his younger self. He doesn't look that different. Hair a bit longer, muscles a bit more defined thanks to his adult body. There's a big difference in their eyes, Sasuke knows. He remembers that all consuming hatred. That need for vengeance that kept him going for a decade. He doesn't feel it now, the gas fueling the fire long since dried up, but he can remember the biting feeling it left in him, the constant irritation and jumpiness he faced when he wasn't working towards his goal.
He pities his younger self. He doesn't know, not yet, what it's like to embrace those around him. He still dismisses Sakura, hates Kakashi, and loathes Naruto. He doesn't see a family the way Sasuke does. He has yet to move on.
When he feels chakra start to spike once again, stances shifting, he knows that now is the time. To change history.
He leaves his hiding spot.