You do realize that all three of them attacked Stain first? It's pretty hard to claim self-defense when you go "I saw these two dudes fighting so I punched one of them."
Yes i do, and no its not really hard to claim self-defence if your laws isnt garbage.
Its called self-defence of 3rd person, and applies when your defending someone who are unable to defend themselves.
Something thats absolutely appliable when a know serial killed is about to shiv somebody.

Also it's pretty disingenuous to say Iida was defending Native when even Stain calls him for focusing on attacking rather than rescuing.
And who gives a rats bum about what Stain think in this case?
Ida "accidentially" stumbles across Stain as he is about to knife someone, tries to stop it.
Thats a story the public would absolutely swallow.

But else yes.
It would be a -lot- smarter if there was some sort of graded system for when and how you were allowed to use your quirk.