Inko marries a charming man with a fire breathing quirk. Even though he works a lot, Inko knows he cares for her. When they have their first child it is the happiest day of her life. She promises to always be there for him, but that promise will be broken. Her husband will make sure of that.
Life have become boring for All For One, so it would be fun to mix things up for once. He decides to obtain a heir leading him to marry Inko. The day his son is born is the day he knows life will no longer be dull.
He spends the next four years subtly training Izuku for his future position and playing the part of busy husband.
Once Izuku is four, afo takes Izuku to his private doctor and has him checked for a quirk. Suprise! There is one. However, afo has the doctor put Izuku down as no quirk, because the quirk he has is nothing like a firebreathing or attract small objects quirk. It's not even a mix of them. Also, little Izuku is told he doesn't have a quirk. Which he is really sad about.
A few weeks later, afo has Izuku and his death faked, but Inko is killed. Izuku is devastated and afo says it's the heroes fault for not getting her sooner. Izuku is even more sad now.
All For One then begins to talk about how the hero system is corrupt and they should do something about it. Like tear it down and recreate it in their image. Izuku eventually agrees with that idea.
And All For One is like, well you're going to need a quirk then. Cue afo 'giving' Izuku a quirk when Izuku actually had it all along. But Izuku is now very grateful to his father and feels indebted.
Izuku is then introduced to Shimura who All For One only took to spite All Might. But afo is like, Shimura here is going to help you remake the hero system. He's basically your right hand man.
Shimura is really confused, but goes with it because afo says so. And this Izuku kid isn't that bad.
So Izuku grows up believing what he is doing is right. And sows a lot of chaos in the process.