The entire manga ending is a burning trash fire that falls on its head in all of the established thematic purposes, but what was done with Shigaraki's character might be one of the stupidest and plain evil writing decisions.
Shigaraki was established as an immature and arrogant AFO's heir and since they were separated after Kamino, his journey was all about learning to stand on his own, developing his own agenda, establishing and chasing the goals he set out for himself. In other words, since AFO's imprisonment, Shigaraki was gradually healing from years of grooming, brainwashing and social isolation.
His backstory revealed in the My Villain Academy arc had challenged the readers and their views of this character, showing his extreme trauma, his guilt as a 5 years old boy who had killed his entire family in a quirk accident. It also finally explained Shigaraki's fervent devotion to AFO, the only one who had reached out, who wanted to help, who cared.
Of course we were aware AFO's motives can't be pure. He was preparing Shigaraki as a future vessel for himself, a means to achieve his dream of living forever.
Then, by the last war arc, Shigaraki has all the agency, individuality, autonomy ripped away by AFO stealing his body. He spends the entire second war fighting for control, silenced by AFO and the narrative barely giving him any chance to talk. One of his main goals was protecting his comrades and their dreams - this is taken from him as well, the League spent the entire second war separated, not even getting one final chance to see each other. Shigaraki dies while thinking of his friends, of his goal to save them, which he will never be able to achieve. While Izuku was at least given a chance to directly confront Shigaraki on his final quest of trying to save him, Shigaraki spent the entire time even before the second war delirious, and until his death, he had never even learned what happened to his friends, the people who chose to follow him, trusted him and supported him.
The very last thing that happens to Shigaraki before his death, is learning that his entire life, everything that had happened to him, was caused by AFO and his schemes. This strips Shigaraki of the last of his agency, turning him into a martyr. His story is no longer one about the unfairness of the heroes-adoring society and hypocrisy, good VS bad victim, trauma and coping with it. His story is about becoming a victim of a creep who had been grooming him since he was 5. One who had tried his best to make a life for himself outside of his abuser, and then had this life cruelly taken from him. Shigaraki doesn't even get to talk or react to the fact he has to sacrifice himself for AFO to die. Izuku doesn't ask him, doesn't apologize to him, he just presents the fact plainly. Shigaraki just has to deal with it.
He went from a victim to a martyr and the story barely even acknowledges it. Deku and Shigaraki were the only people who had learned the truth and neither talks about it or is allowed to share this with the world. Shigaraki died an unwilling martyr, and is remembered as a monster no better than AFO. The public draws no line between the abuser and his victim. Izuku or Allmight don't exhibit any intention to reveal the truth, and we already have Dabi's example on how this turns out.
What is the point of this character ending? Life is unfair and everything is horrible? Just deal with your trauma without causing problems for others, or you will be punished? Always abandon your ideals and your friends if a cop tells you to, or you will be murdered?
