I don’t understand. You had this perfect cast of Kind of heroes opposing the real Bad guys which were the Burning Legion and the Scourge yet for some reason we kill them all because Blizzard’s too lazy to create new named bosses and started to burn through the whole old Warcraft cast?
Make it make sense.
Well, Blizzard was going to add Illidan to the Horde, but then they forgot to do that and just went straight to killing him and his allies, as is tradition with all Horde characters.
Because like with any situation in Warcraft, we go in bows blazing without asking any clarifying questions whatsoever to create way more problems than there already were to begin with.
It’s like some weird fantastical episode of Seinfeld where a simple conversation could’ve solved everything but then the episode would only last like 5 minutes.
I’m not sure why we’re asking questions to answers we already know. Blizzard had no prepared follow up to Classic and had to hastily create an expansion with little time for long term effects of their design and narrative choices, while actively (though slowly), walking back many of the choices they made at that time as we see with the ‘redeemed’ Illidan, the Revendreath Keal, and Maldraxxus Vashj.
Yeah this one makes the most sense.
Blizz agreed which is why they brought him back
Illidan was never evil.
do first , think later.
Marketing.
He has done some messed up things, even in his novel. Saying that Illidan was “never evil” feels wrong.
Was never a good guy you meant to say. He’s done plenty of outright evil things
Friendly reminder that Illidan was hiding in Outland because he failed Kil’Jaeden. Illidan being singularly driven to end the Legion didn’t become a thing until the Legion expansion.
His list of crimes include: Attacking Night Elves for using Malfurion as a reason for objecting to the 2nd Well of Eternity’s creation, burning down Villages to keep anyone from following his boat ride to the Broken Isles to get something to destroy the Lich King in order to avoid assassination by Kil’jaeden, ordered Kael’thas to invade Shattrath only to not even enforce the order once the Scryers defected on moral grounds, plundered the Draenei Tomb Auchindoun of Souls and finally enslaving a bunch of Broken to claim Outland’s resources!
Maiev his jailer also has a rap sheet: Abandoned Tyrande to be carried downstream and falsely accused Illidan of Tyrande’s death in order to continue chasing Illidan, assassinating Shen’dralar(who fought against the Burning Legion unlike the Highborne of Zin Azshari) who the Night Elves welcomed into their ranks and attempting to murder her own brother Jarod as well as Night Elven Leader Malfurion.
Tyrande has crimes: massacring a group of Wardens to release Illidan.
That’s 3 noteworthy Night Elves who massacred a lot of people.
Malfurion and Jarod on the other hand are innocent compared to those 3!
Illidan of course would be the worst of the 3 as he started enslaving Broken while using Draenei Souls from Auchindoun to fuel a Portal to Mardum on top of burning those Villages down!
It is also telling that despite the Infinite Dragons being the reason we go do the Battle for Mount Hyjal raid, we never see any infinite dragons in the raid itself.
Most of TBC was “rule of cool” basically. Aka “wouldn’t it be cool to fight these legendary characters from WC3!”.
He ruled over Outland with an iron fist.
The Sons of Lothar and the Draenei didn’t like that.
As for the Horde, Silvermoon betrayed Kael’Thas… or the other way around? I don’t know anymore. Silvermoon was allied with the Horde, while Kael’Thas was allied with Illidan. Casus Foederis
The first enemy was the legion.
Honestly, Kargath might be more egregious as his traditions are part of Thrall’s horde with his name attached to several places including the town of Kargath. 
Besides some smaller details, the mag’har are enemies to the illidari while the blood elves want to help the draenei. 
Illidan was presented as the main villain of BC because when Blizzard created the expansion, they thought it would be cool for players to fight against characters like Illidan and Kael’thas that they played as. From that point, lore was crafted in witch Illidan had lost his mind from losing to Arthas* and him oppressing the people of Outland.
Kind of good Guy. Not a hero sure but still tried his best to defeat actual evil people.
It kinda pains me to say this but I always liked to think that when Illidan lost to Arthas, part of his soul was taken by Frostmourne. As a way to explain why Illidan was insane in TBC.
Shadowlands does kinda support this headcanon given that it says that Mourneblades render the souls of its victims in two. Maybe as a half demon, half night elf a fragment of his mortal self was taken. Leaving only the demonic part left. This imbalance is what causes him to mentally lose it.
Obviously the Illidan novel says otherwise but prior to that, that was my headcanon as to why Illidan was the way he was in TBC.