I don't think we should focus on whether what he did was 'good' or 'bad'—because it's neither.

To preface this:
I don't agree with this choice either. I don't like that Horikoshi chose this as Izuku's conclusion. It's good in theory, but the execution is questionable. I think that it abandoned the themes of the story.

To my actual answer:
Izuku isn't obligated to say yes to Katsuki's proposal to join his agency. He is within his rights to say no.

The war left Izuku with unresolved traumas (another one of my problems with the writing). It changed him fundamentally as a person, and thus his sense of heroism has changed.

Katsuki recognizes all of this and respects Izuku's decision.

A lot of the fans' anger is misdirected. If anything, we should be upset at how things were written to play out, not Izuku himself. Again, Izuku has a lot of unresolved traumas, hell he called himself a murderer in the last 5 chapters. Does that seem like a person who'll feel 'deserving' of being a full-time Pro Hero again?

The extra chapter fucked everyone over. Izuku, Katsuki, and hell, even Ochako.

Why do we as a fandom always extend grace to Katsuki, but never to Izuku?