Izuku's backstory, full-stop. The day he developed his Quirk, he tried to excitedly show it off to his parents, only to end up starting a fire that spiraled out of control and killed a third of everyone in Musutafu. While the court acquitted him due to being a child and because it was a complete accident, everyone still hated him to the point they tried to kill him in retribution for the deaths of their loved ones. Despite the Bakugous attempting to protect him, he let everyone dump their hate on him as he believed he deserved it. Then, one day, he was kidnapped by a facility full of people who wanted revenge on him and was subjected to Cold-Blooded Torture. His captors even raped him and the trauma of it all caused his Quirk to mutate fully, turning him into a Draconic Humanoid that only made his self-loathing worse. Even after being rescued, he still holds deep trauma and self-hatred, believing himself beyond redemption for the accident he caused all those years ago and that he deserves to continue suffering.
Hana may have been resurrected and empowered, but she's still deeply affected by the events of the day she died, viewing the betrayal of her bother as her My Greatest Failure moment, and seeing her formerly hero-worshipping brother as a ruthless villain devastates her.
The confrontation with Shoto showcases the depths of Izuku's depression and loneliness; he remarks that he barely remembers what his parents look like and honestly admits he'd give anything to see them for just a split second: He emphasizes dying would be perfectly fine if that's what it'd cost. He also mentions how no one could love a monster like him.
As vile as he is, there is no doubt that All For One loved his daughter and misses both her and her husband greatly; even having Ujiko try and bring them back to life... but with the express purpose of reuniting them with Izuku. There is no hint that he plans to reunite with them at all, and even his interactions with Izuku are quite limited.
The unquestionable love and adoration for his grandson are both this and Heartwarming; Izuku is aware of who his grandfather is, but seems to care for him. And All For One can never really be with his grandson but is always watching from afar.
Then the fact that, even with Nine's body, he and Ujiko know he's not long for the world. To see such a ruthless supervillain being so accepting of his own impending death can be... sobering.
