Hey, remember when the Illidari were the bad guys?
While the focus of the expansion eventually shifted to the Burning Legion and its third and ultimately most underwhelming invasion of Azeroth (unless you count the Battle of Undercity as its fourth,) the Burning Crusade was primarily concerned with Illidan's forces and their domination of Outland.
Legion as an expansion has done a lot to rehabilitate Illidan as more of an anti-hero than true villain, but he is a character who has really straddled that line.
I've remarked before that the irony of WoW's "Hero Classes" is that they would be more accurately called "Villain Classes." There are standard classes that deal with unsavory methods (Warlocks in particular, though Rogues probably count too,) but both hero classes are members, or former members, of organizations we have actively fought in the past.
Death Knights, at least until Legion, had a bit more of a black-and-white switch between good and evil. Upon their raising, they served the Lich King directly and slaughtered civilians - there are quests where you actually get sent to kill fleeing humans who are about twenty levels below you (one of the rare times when character level is actually lore-based.) You are a straight up bad-guy, a monster. But at the Battle of Light's Hope Chapel, it's not exactly explicit, but it's pretty clear that your free will is restored, and from that point on, the Ebon Blade fight for the living.
(Yes, things have gotten muddier in Legion, but let's set that aside for now.) The Ebon Blade even attempts to make amends by allying with and bolstering the Argent Crusade, and it's their combined efforts that allow the heroic adventurers to secure victory over Arthas. The Ebon Blade is aware of its sins, and at least at first they were interested in paying their debts.
The Illidari, though, are tougher.
Part of the problem is that the Illidari never questioned that they were the good guys. The Scourge was pretty aware of their own evil. There's actually a really fascinating lore book in Acherus that you only get to read during the starting experience, but it's kind of a political manifesto by Kel'thuzad, who talks about how the Scourge considers itself to be more amoral than evil, but that they use the imagery of "evil" as an intimidation tactic.
But the Illidari have always considered themselves to be the ones who sacrifice everything, including their reputations, for the greater good. Their goal is to keep Azeroth safe from the Legion. It's not about their goals - it's their methods.
But let's talk about those methods.
First off, there's the use of fel magic. Now, most Demon Hunters do really seem to use this power for good - fighting demons. But we've seen multiple cases of Demon Hunters going over to the dark side - Varedis and Calia Felsoul, and that one in Azsuna. It's surprisingly easy for a Demon Hunter to just flip and join the Legion. On top of that, every Demon Hunter (with the exception of Illidan, who's kind of his own thing) actually has a demonic soul bonded to them, and if they ever let go of their vigilant control over that demon, they will be taken over and, you know, just become a demon.
In the long run, that seems kind of inevitable. Granted, if a Demon Hunter can kill a bunch of demons before they lose control, it might be worth it as a sort of net effect, but only if they can really, truly kill the demon, not just kill their physical bodies.
We've also got to go back to BC and look at what they did in Outland. Now here, there is a little leeway we can give them, as the Illidari were a somewhat fractured faction, with Kael'thas' forces allying with the Legion in secret. Likewise, Vashj may have been pursuing her own goals on the orders of Azshara.
But Illidan himself did some pretty nasty things. For one thing, he created a whole new Fel Horde, installing Kargath Bladefist as their Warchief. The Orcs, who had on Azeroth finally escaped the Blood Curse, were on Outland being pushed back into this old habit. And it's clear that a lot of Orcs were being forcibly infused with Magtheridon's blood.
The Orcs in turn enslaved the Netherwing dragonflight - the off-shoot of the Black flight that seems to have escaped the Old God corruption the rest of Neltharion's brood had been afflicted with (and the Orcs sold many of those Nether Drakes back to Sinestra, which led to the Twilight Dragonflight.)
The Draenei had managed to retake the ruins of Shattrath City, but Illidan sent his forces to attack them. Why? Just so he could consolidate power on Outland.
Now, yes, Illidan was working on a plan to invade Mardum and then Argus, and killing him might have had seriously bad longterm consequences (which we're in the process of correcting by fighting to his body in the Nighthold) but the thing is that he never told anyone what he was doing, or even that he was actively trying to fight the Legion.
There was no reason for any of us in Burning Crusade to think Illidan was anything other than a tyrant who needed toppling.
And player Demon Hunters were totally complicit in this.
The Demon Hunters were locked up in the Vault of the Wardens for ten years, but they've never really owned up to any of the bad things they or their boss did. Or even if they've owned up to them, there's never been any sense of remorse. Much like Illidan, the Illidari are basically entirely self-centered and self-righteous.
But they're a really fun class to play!