I'd say it's how the Imperium swallowed up and destroyed a number of civilizations that had separated from them that had been developing in much more progressive, prosperous ways. The Olamic Quietude and the Interex come to mind as examples. They showed that humanity didn't have to go down the terrible path they've ended up on.

Or, going farther back to look for a single "worst thing" that's had the greatest awful knock-on effects, I'd say that'd be the Old Ones' refusal to grant any aid to the Necrontyr when they asked for it. That one selfish act sparked off the War in Heaven, created the Chaos Gods, and everything that followed.

If you can't find the books available through legal channels in your country, you might want to consider looking for them on the high seas. !piracy is a good resource for that sort of thing.

When you buy your first model. That event represents your transition from "I'm interested in Warhammer" to "I collect Warhammer."

Worst thing overall was probably the fall of the Eldar. They created the 4th Chaos God and tore a hole in reality straight to Hell across half the galaxy. Screwed things up bad for everyone.

Most tragic, my vote gets the fall of the Thousand Sons. They would've been the most potent anti-Chaos tool the Imperium had by far, and seemed to genuinely care about the humans around them and their cultures. Magnus was extremely prideful but well-meaning, which led to him making deals with Tzeentch that doomed everyone.

Most disturbing, I go with the child who's favorite toy was a servitor dressed like a clown. He was a psychotic murderer who was turned into a lobotomized cyborg toy-slave, and the kid thinks of it like a teddy bear. Servitors in general are messed up.

Probably the creation of Orks.

I subscribe to the idea that since Orks are living in a utopia from their perspective, they are the only faction living in utopian circumstances, and we know Ork psychic energy can affect reality so the sum total of all Ork desires may effect the entire universe, the current circumstances in the 40k universe are all a consequence of Ork desires. Every bad thing that has happened since Orks were created were to engineer a paradise for Orks, which is an irreconcilable deadlocked state of total war.

The Horus Heresy. When you see what the Emperor's vision actually was, and really get to compare it to the galaxy they got, it's just... so much worse. And the galaxy is pretty goddamn bad to begin with. But you know there's a special place at the very bottom of hell for Erebus.

Of all the people that could be blamed for how bad things are, I think Erebus is the one who deserves it the most because he knew what he was doing and acted out of malice in a way that others didn't. Magnus and the Emperor did a lot of bad shit, but they did it for "good" reasons.

Keep in mind that even "good" is extremely subjective. The imperium's goal before the heresy was to eradicate any and all Xenos, which is definitely not good. However, just before the heresy began, there was a glimmer of hope that maybe the IOM would take a less hardline approach toward Xenos if they could establish diplomatic relations. Any hope of that was ruined though once the HH began.

Edit: Also, if you are looking for ways to access the books, I'd recommend sailing the high seas

Here's a bunch of books you can download. If you want to learn lore through a game Rogue Trader is great, same kind of RPG as Baulders Gate or Wrath of the Righteous.

I'm not a huge Warhammer nerd but I've been playing/interacting with the lore since like 2000. Couldn't possibly say the worst thing that's ever happened. The Warhammer universe is tens of thousands of years of tragedy and carnage.

The all guardsmen party is fucking fantastic. It's actually the story of someone's campaign in the Warhammer RPG. It should give you an ass ton of background lore, as well as an idea of how the game can work. It's also probably the funniest thing you've read in a long time.

Edit: Perhaps I am warhammer nerd.

Most of the lore is written about the Horus Heresy. From the point of view of humanity, it's the worse thing to happen to the imperium, as it has not and will never recover from it.

There's another human civilization called the interex that was basically the anti-imperium in that they were pluralist, secular, and sought peace within the galaxy. The imperium destroyed them, so it pretty much crushed any alternative system or hope in the universe, so that was pretty bad.

As far as actual body counts and gorey details? I don't know, the eldaar, dark eldaar, old ones, and necrons are all pretty fucked up.

https://annas-archive.org/ https://libgen.rs/

Kinda OTT: what videos have you been binge-watching? I don't crap about Warhammer, but I saw this and it's epic:

Astrates

I mean, the war in heaven was the most cataclysmic event. It's sort of the first galactic scale apocalyptic event in the lore. Gods were fighting gods, some races rose in power, while others ceiced to exist. It's speculated that at some point the nature of the warp changed, and was later distanced from baseline reality.

But this is all background to the lore, and none of it ever happens in the books, so I'm not sure it really counts. I think the same could be said for the fall of the eldar.

So probably the horus heresy or the 13th black crusade. My money's on the latter as it caused the great rift.