Illidan Stormrage is at the center of the Legion expansion, despite the fact that he has spent the entirety of it so far dead. At our hands. Yes, Illidan was the headliner to the Burning Crusade expansion, and indeed much of the expansion felt like a build-up to our ultimate confrontation with him, though Blizzard was still figuring out expansion pacing at the time (and given Warlords of Draenor, still screws this up on occasion) and so the raid with Illidan was actually released in BC's first major content patch, which led to a need for the Sunwell Plateau raid that came if I recall correctly in early 2008.
As a newcomer to the Warcraft universe when the Burning Crusade expansion was launched (I had only been playing WoW for a couple months and had not yet played any of the RTS games,) I really didn't have a sense of who Illidan was, and in fact sort of assumed that he was the ultimate villain of the games based on the expansion's launch trailer (I knew he was a demon and only learned far later that he was ever not one.)
In all honesty, the storytelling in Burning Crusade was not terribly clear - Blood Elf players would level up through their starting zones as loyalists to Kael'thas, and even get to Hellfire Peninsula still believing the propaganda that Outland was some paradise, and there wasn't really a clear moment when it became clear that Kael'thas had sold out to the Legion - we just kind of started fighting his followers when we got to Netherstorm. Likewise, it wasn't entirely clear what relationship the Burning Legion had with the Illidari forces - for example, the Fel Horde under Warchief Kargath Hellscream was loyal to Illidan (who had provided them with Magitheridon and his potent Pit Lord blood) and yet also seemed to be working with the Legion proper.
The image that we get in Legion is one in which Illidan is clearly a hero, even if he is an anti-hero. Yes, he has done some terrible things, but all in the interest of saving Azeroth (or even just Tyrande.) We get an image of Illidan as someone who has dedicated everything he is to stopping the Legion, and indeed creating a new class that was founded on that principle (the folks at Blizzard Watch pointed out something really cool about the scenario at Black Rook Hold that is part of the Light's Heart quest chain, which is that as you play as Illidan when he was a mage in the early stages of the War of the Ancients, the various techniques he unlocks after draining his followers are actually just Arcane versions of various Demon Hunter abilities - clearly Illidan took these techniques and adapted them to use Fel magic.)
In fact, during the Light's Heart quest chain, Xe'ra actually speaks about you, the player, being the one who needs redemption for killing him (I've completed this chain on my main, who has done Black Temple, and my Demon Hunter, who actually has as well, but apparently the class-specific quest text took precedence, as Xe'ra made no admonishment. Given that any Demon Hunter's clear of Black Temple is automatically non-canonical - as at that very moment your character is doing the starting experience on Mardum - it makes sense that they wouldn't blame you for Illidan's death.)
Now, here's the thing: was our act of slaying Illidan Stormrage in Outland really wrong? Certainly, given the foretold role he will play in defeating the Burning Legion, it may have been ultimately ill-advised, but was Illidan really just a good guy whom we were too blind and self-righteous to see for what he was?
I think the first thing to consider is that if Illidan had a secret plan to defeat the Legion and he wasn't sharing it with, say, the Draenei, that was a pretty serious oversight. Illidan was exiled after the Third War, and he did have plenty of reason to distrust his fellow Night Elves, but surely the greater Alliance and the Horde were more likely to hear him out. After all, at this point Warlocks had been contributing to Alliance and Horde victories for a few years.
Instead, Illidan decided that he would rule over Outland. He did some important services, like closing the portals left open by Ner'zhul and wresting control of the Black Temple away from Magtheridon. Why, then, did he attempt to conquer Shattrath City? The Scryers were initially a contingent of Blood Elves under orders from Illidan to take over Shattrath, who instead turned after their leader experienced a vision from the Naaru.
In fact, it was the Naaru who sent us to deal with Illidan in the first place, which makes the admonishment from Xe'ra feel particularly unfair.
Beyond helping to create a new batch of Fel Orcs and acting like a despot in Outland, we also have to talk about the events of the Frozen Throne.
Now, aside from the Founding of Durotar campaign, no one really winds up looking great in the Frozen Throne expansion of Warcraft III. Actually, maybe we should just say that no one winds up looking great in the Frozen Throne. Ironically, if there's a clearly heroic character in it, it's probably Kael'thas Sunstrider. Many WCIII players were shocked to discover that Kael'thas had become one of the most vile monsters in the Burning Crusade expansion, given that in the game in which he was introduced, he seemed to have nothing but noble motivations.
Still, the main conflict in the Frozen Throne is that between the Illidari and the Scourge. Illidan manages to take over Outland, but almost immediately he is confronted by Kil'jaeden, who tells him that Illidan must go and destroy the Lich King or face immediate eradication.
So Illidan's motivations for taking on the Scourge are not exactly noble - he's not trying to save Azeroth from the undead menace. He's mostly just trying to save his own hide, and is actually doing the will of one of his main supposed adversaries.
Did he have a choice? Probably not. Still, this is collaboration, not resistance.
In fact, when we come to Outland, it appears that Illidan is in a total panic, with the Legion closing in on him (when his forces don't seem mixed up with theirs, as I mentioned earlier.) During one quest chain in Shadowmoon Valley, Illidan makes one of his rare appearances, yelling out that not even Arthas could defeat him, and thus we wouldn't stand much of a chance.
Which is of course just plain incorrect. By any measure, Arthas did defeat him. Because Arthas went on to become the Lich King while Illidan's forces beat a hasty retreat to Outland.
So it does appear there are some inconsistencies. The Illidan portrayed in the Burning Crusade (albeit just barely portrayed at all,) was something of a gangland kingpin whose empire was collapsing, not the all-business man-with-a-plan badass that we're seeing portrayed in Legion.
It's been nearly ten years, and I don't blame Blizzard for wanting to do a little rewriting of the story, especially if they want to make Illidan a central part of this expansion. And to be fair, it's not like they're suddenly making him into Mr. Nice Guy. While they're shifting him from villain to anti-hero (a fence he was already kind of straddling) they're not suddenly saying his heart was as pure as Uther the Lightbringer's.
But I hereby throw a little sass back at Xe'ra for being so judgmental about our raid on the Black Temple. Illidan might not be the most deplorable villain we've ever faced, but I don't think that we should look back on everything we did in Burning Crusade as one huge mistake. Ok, maybe Zul'Aman. That was purely just invading another civilization's capital and killing its people for pants.