Watching his wife lying lifeless in his arms, Uchiha Shin cried out in anguish.
"Ah! Ah! Ah! Kaori, don't die! How could this happen? This isn't real! That's right! This isn't real! Dad! This is all just a genjutsu, isn't it? Everything that's happened is an illusion you created, right?!
"That's right! Creating a genjutsu this realistic would be easy for you, wouldn't it? After all, you have the Mangekyō Sharingan! Did I do something wrong, Dad? Why are you punishing me with a genjutsu? Please, dispel it!"
Uchiha Setsuna had his Mangekyō Sharingan activated as he looked down at Uchiha Shin, who was clutching Uchiha Kaori and weeping.
Uchiha Setsuna had awakened his Mangekyō Sharingan in prison.
Back then, he'd thought he would spend the rest of his life in prison, unable to lead the Uchiha Clan to reclaim its former glory.
That had filled him with resentment toward Tobirama Senju for locking him up. He also regretted attracting attention by making too much noise. Otherwise, he would have succeeded in helping the Uchiha Clan seize control of Konoha.
So Uchiha Setsuna had spent his days in prison cursing Tobirama Senju and regretting all the things he should have done differently.
Under the prolonged strain of such volatile emotions, Uchiha Setsuna awakened the Mangekyō Sharingan one day.
They said the Sharingan was the eye that reflected the heart, and that the Mangekyō Sharingan's abilities were tied to the user's state of mind when they awakened it.
Thus, Uchiha Setsuna had awakened two abilities, both of which reflected his state of mind at the time. (I can't be bothered to come up with names for Uchiha Setsuna's Mangekyō abilities.)
If only he'd been stronger, the Anbu wouldn't have caught him. If only he'd been stronger, he could have defeated Tobirama