Admittedly, my criticism is based on the "outrageous" things people mention on reddit and elsewhere, but it really sounds as if the Horus Heresy is its own universe, "with its own rules and logic"
Well, I'm just a reader without any insider knowledge (but I've read almost all HH books), and for me it's looks like they have had an idea: "Let's tell the story about how bad guys are not so bad and good guys are not so good". The problem is that you need a really good authors for such a goal, but the quantity of good authors in BL is something around 0.5 - and they failed. Anything written good introduces "its own rules and logic" and anything written bad is... just bad.
ADB pulled it off... to the point half the fanbase wonders if the guy has father issues, the way he writes the emperor.
He really turned angron from burmmaimkill* into a broken figure that's actualy making a good point about the great crusade being a form of conquest and slavery ("join us or die", isn't exactly a choise is it?)
* yes I know that's Kharn's line.