This article contains SPOILERS for My Hero Academia chapter 364!
All For One just heavily implied why he chose to fuse his other self with Tomura Shigaraki, and the implications of this revelation are much worse than My Hero Academia fans expected.
Mangaki Horikoshi initially flabbergasted readers when All For One first began to absorb his protégé way back in chapter 295 at the end of the Paranormal Liberation War, creating the impression that he was only raising Shigaraki to be his vessel. Then things took a strange turn when My Hero Academia later revealed that there were somehow two All For Ones, where the true All For One had likely created some sort of clone for the sole purpose of fusing with Shigaraki.
Now in chapter 364 of My Hero Academia, the truth may have finally come out. All For One just used Eri's Rewind quirk on himself to revert back to his physical prime, so he can battle Endeavor at full capacity. But as the villain reveals, once activated, there is no way to stop it - Rewind will continue to have an effect on him until it kills him. In other words, triggering Eri's quirk on himself means All For One is sacrificing his life to do as much damage as possible before he passes. Interestingly, All For One says that with his death, he's passing his dream onto Shigaraki, but is that really the case?
It's not clear what the All For One/Shigaraki fusion will look like, but it's obvious from his previous statements that All For One plans on becoming the dominant personality. In other words, All For One never intended to pass the baton to Shigaraki. When he began planning as many contingencies as possible for this final battle, All For One obviously projected that a possible scenario could result in him having no choice but to use Eri's Rewind on himself. Since this would eventually end in his own demise, it's likely that All For One planned accordingly by introducing this fusion alternative so that he could live on in Shigaraki's body after death. He therefore isn't passing the baton over to Shigaraki - he is passing the baton to himself, having claimed Shigaraki's form.
While Shigaraki originally believed that All For One was fusing with him to make him more powerful and similar to My Hero Academia's supreme villain, it turns out that Shigaraki's entire life has just been a contingency plan that All For One intended to activate if he got overwhelmed fighting the world's heroes. All For One explicitly doesn't trust Shigaraki to carry on his aspirations, but rather always saw him as a bolt-hole in case of emergency - indeed, had All For One not been pushed to employ Eri's Rewing, Shigaraki's role in his plans would have been greatly diminished. My Hero Academia continues to stress the horror of All For One's disregard for Shigaraki's humanity and agency, as the villain proves he's committed to fighting to the last.
My Hero Academia chapter 364 is available to read now from Viz Media.