I find the idea that the Emperor censured Monarchia with no warning extremely implausible. Lorgar (and certain fans) like to act as though it was a bolt from the blue but fuckin...I doubt that very much.
That has the same vibe as shitty husbands who say the divorce came out of nowhere, when in reality their wife had been trying to talk to them for like 5yrs before pulling the rip cord. That's painfully self-centered, entitled asshole talk and we know that's extremely on brand for our boy.
Logic dictates a few different things.
Lorgar knows damn well the Emperor hates religion, he knows his father is not on board with the proselytizing, that he's repeatedly and firmly stated he's not divine. Lorgar knows what the Imperial Truth asserts. He's just decided he knows better, which is wild behavior from a guy who insists he lives to serve his god. Lorgar loves talking about submitting to the will of a higher power but if he actually gave a shit about that, it would look like him keeping his faith to himself and maybe, at most, practicing in private. Yet he doesn't, because he has such a monstrously huge ego, it allows him to believe he knows better than the man he views as a god.
He knows what the deal is, but he decides to do his bullshit anyway and then pulls a surprised Pikachu face when it blows up in his lap.
It's not because it genuinely came without any warning, it's because Lorgar is an entitled narcissist who has been operating under the belief that the rules don't apply to him; he's a special exception and he's correct, so he gets to do what he wants.
He's shocked by the destruction of Monarchia because his mind could never have conceived of a scenario wherein he isn't granted full carte blanch to do as he pleases by virtue of the fact he's the special son of a special man. You see this with how he later interacts with the Custodes after Monarchia; he has mini-tantrums at them when they do their babysitting duty, as if he thinks it's reasonable to believe the beings programmed on a molecular level to obey the Emperor's every word will defy the Emperor simply because Lorgar commanded it.
Even when Malcador shows up to give him his official face-to-face warning, Lorgar is still going full-blown "MY FATHER WILL HEAR OF THIS". Meanwhile, anyone with a lick of common sense would be able to deduce that Malcador isn't here just because he feels like making Lorgar's life harder. Lorgar's brain won't let him accept that, so they literally have to bring the Emperor down in person to put the boot to his neck. That's the extent they have to go to to finally penetrate his narcissistic delusion. Hell, even then, he's still arguing with the Emperor until his father has to use psyker abilities to make him shut the fuck up. It's honestly insane how resistant Lorgar is to being told to cut it out.
It makes you wonder how many other times in the years preceding Monarchia that Terra tried to talk to him while he just went "Mm...nah, no I don't think so :)"
Also the Emperor is consistently reluctant to throw Primarchs in the trash. You can be pretty fucked up and he'll still figure out a way to use you. See: Angron, Curze, Mortarion, etc. He'll find workarounds. He'll give chances. Even towards the end of the Heresy, Sanguinius is pretty sure the Emperor would forgive Curze, even after everything he's done. It takes so much shit for Big E to finally order Curze's death.
And in no way am I saying this is driven by paternal sentiment: the Emperor spent a lot of time and money and a chunk of his soul on creating the Primarchs, he's already had to kill two, he's not eager to throw away any more priceless resources.
So it should be very telling that the Emperor was going around in the lead up to Monarchia, asking the other Primarchs if they believed he should just say fuck it and kill Lorgar.
This tells me that efforts had been repeatedly made to get Lorgar to shut up and sit down over the course of 200 years, but nothing was working. If anything it speaks to the Emperor's patience; he'd probably been sending "Lorgar, stop it" emails for decades.
The Emperor considering making Lorgar a third Lost Primarch implies he was beginning to suspect there was no way to salvage the situation, that he had to go big as a last ditch effort.
Which ultimately makes Lorgar even less sympathetic. He likes to pretend "Woww Terra allowed me to think I was doing right by the Emperor for years, only to humiliate me and my sons for no reason without any warning. I'm being abused. This is abuse."
But if you stand back to look at the situation objectively, there's no fucking way it actually played out like that. No shot, dude.
EDIT: just remembered a part in The First Heretic where one of the Wordbearers--maybe Erebus--mentions a past compliance and calls their proselytizing "spreading the Imperial Truth"
Which is wild because we all know that is not the Imperial Truth in 30k, it's the exact opposite of the Emperor's doctrine, and they all know that. It really showcases his narcissism; Lorgar's been running a cult with himself as its central figure the entire time. Within the only group who views the Emperor as a god, Lorgar's word takes precedence over his. Lorgar solely views the Emperor's "divinity" as a tool, it's a means to lend himself credibility and bolster his greatness. He's doing a juiced up version of namedropping.












