Itachi dies in the 4th Shinobi war.
After the debacle of his first death, he is blindingly happy when he is forcefully reincarnated and is able to aid his little brother as he always yearned to.
His (second) last moment is spent reconciling with Sasuke, and that alone is worth the pain of the unnatural state his soul is twisted into.
After he is sealed, he spends an indeterminate amount of time … floating. Time is immaterial in this place, and he doesn’t spare it much thought.
Without a body, he is free from the neurochemical pitfalls of his psyche. The self-loathing, crippling anxiety, over-inflated sense of responsibility - those are all the trappings of the living. His grief is still with him, however, as well as a myriad of regrets he collected over the years. A truly staggering amount for such a scant handful of years. But they are… muted. Grief fades over time, and the Void is nothing but time.
With a strange sense of detachment, he comes to consider his life as if it were a stranger’s. In a way, it was. He is no longer that terrified, brainwashed child, nor is he the insane, traumatized thirteen-year-old mass-murderer.
Detached as he is, he is taken aback at the rush of outrage that washes over him, entirely on little Itachi’s behalf. Emotions are hard to come by in the void so he savors each of them, as he studies the tragedy that was the Uchiha heir.
Anger spikes red when he thinks about the crude manipulations of the Uchiha clan. Helplessness and pity swirl together when he considers the Clan’s situation - a disastrous tale of a proud family beset on all sides. Their Sharingan induced trauma didn’t help, and neither did the obsessive love and possessiveness towards their loved ones. Rage quickly joins in, as does bitterness, but anguish overtakes all other emotions soon enough.
He is, all told, anguished over little Itachi. The innocent child, traumatized by the war he should never have been exposed to, was then promptly indoctrinated by Konohan propaganda. At four years old, reeling from the bloody reality, he was taught to equate safety with peace, and peace with the wellbeing of Konoha. And if the nebulous concept of Peace didn’t include his Family, well… In the end, only Konoha mattered.
For indoctrination to work, fear is required. Fear and intelligence. Whatever else might be said about him, little Itachi was a genius without equal. And geniuses aren’t known for their emotional stability, even under optimal conditions. Pacifist geniuses whose soul didn’t match their fate were easy pickings for the war machine that was a Hidden Village. At six years of age, he was as devout a fanatic as could be.
The only flaw occurred when his indoctrination clashed with his inherent Uchiha madness.
Really, Void-Itachi mused, that’s why the Uchiha weren’t used as the perfect army. Objectively speaking, they were a clan of warriors, generations upon generations of people selected for their lethality. As a group, they were more intelligent, physically superior, and had the ultimate fighting Doujutsu. They were also deeply traumatized, almost to a man. Sharingan allows but also forces you to relive your most traumatic experiences. Perfectly. Forever.
Using it in combat means obsessing every clash, every spar, every experience other combatants might want to forget. It makes for unearthly combat experts, that are almost destined to go mad with grief and trauma. That, in turn, makes them easy to manipulate and lead. In theory.
As most would-be world leaders in the Elemental Nations soon realized, you cannot, in fact, effectively lead the Uchiha. While it’s true that their biology makes them malleable, it also protects them - in a matter of speaking. Namely, every Uchiha has at least one person they love more than life itself. Because of that, leading the Uchiha will only be possible until your goals don’t clash with the interests of their People.
With that in mind, the case of one Uchiha Itachi is particularly interesting. There was a genius on par with Uchiha Madara, who was a textbook-perfect example of fanaticism - up until his little brother was born. Then it all got twisted together, his fanatical love for his brother clashed with the fanatical love for his country, and little Itachi’s mind broke under the strain. What remained was a broken shell of a human, one step away from an automaton, waiting to die, while trying to accomplish his earlier goals without really being aware of what those goals were.
He thinks about people hurt by his actions - Sasuke, Shisui, his mother and father. He thinks about all the pointless sacrifices. The pain and madness and broken thoughts. He thinks about the nature of responsibility and the nature of truth.
He evolves. He accepts and deals and lets go. He floats in the feeling of peace and serenity, thinks about his life, and forges himself anew.
Then, he wakes up, cradled in his mother's arms, looking at the blurry faces of his parents, as Fugaku proudly names him Itachi.