I would have to vote for the Cypher/Luthor/DA fluff as well. They went traitor, but repented, while the Fallen who are loyal are hunted because they know this secret which could destroy the now loyal chapter. Kind of a parallel to the Alpha Legion story, except they may or may not be loyal still.
Re: The primarchs immaturity, bear in mind they are to all intents and purposes brothers. I have six, highly intelligent, sensible and educated elder brothers but when you get them together the immaturity is unbearable sometimes. Not hard to see the primarchs acting like that amongst themselves, even if they do appear noble and sublime to others.