"We're seeing levels of retcon and gaslighting that shouldn't even be possible..."
Jumping Jehoshaphat, what a load of crap this guy is piling on! Unbelievable. If you look at the development of the lore, and understand the people who were writing it to begin with, the lie becomes very clear. No, the Emperor was not "always a bad guy". The whole franchise was first created by avowed leftists, who were trying to push a certain idea. Namely that religion is utter crap, conservatism is terrible, and that -- get this -- the Emperor was a wise, progressive atheist who tried to make humanity "better", Star Trek style. But he was betrayed, and chained to a throne in an undead state, and is doomed to suffer forever as his legacy is warped into the exact thing he hated.
That was the original idea.
Then, it turned out that the target audience actually disagreed with the basic assumptions, and concluded that the Empire was pretty cool, actually. Doing the best thing possible in a shitty situation, as a matter of fact. And GW could either keep pushing their own political message and alienate the fans, or go all on on "Space Marines are cool! FUCK YEAH!" and keep raking in the cash.
They chose the latter.
Trying to re-imagine the Emperor as "always a villain" is really, really ironic. It doesn't fit the orginally intended narrative, nor does it fit the narrative that evolved over time due to fan feedback.
If anything, the big take-away from the narraive that evolved over time is that the Emperor is a tragic figure. A superman who did his best, but who was still "all too human" in his soul. He didn't try to stamp out religion out of hate, but because he wanted to starve the Ruinous Powers. It didn't work, because humans don't really work that way. They find something to believe in. And in a twist of fate, they found him, and believe in him, and that actually does work. That does what his orginal plan couldn't achieve. It keeps their souls safe.
Which is a really cool parallel to how the original lefist assumptions GW tried to peddle could never really work, but when they dropped that and started treating the Imperium as the cool faction it is... that did work.
(And lefties fucking hate that. Which is why they want to retcom the Imperium into villains, the Emperor into a monster... and want to gaslight you into thinking that was always how it was.)