He's an anti-hero and his design is cool. That's why people like him.
Well, to be honest, people like characters like Illidan because these are rather rude than polite, generally "don't give a fuck" and do whatever they want. It's pretty common to glorify rudeness and selfishness as "display of strength".
Obviously a good storytelling includes a more complicated character that isn't just black or white, but generally audience seems to like characters who on top of whatever motivations they have, also behave like jerks. And then you wonder why people are jerks for no reason.
I'd wish to believe some characters are cool despite being jerks, and not because of, however I'm not so sure anymore.
The only tragic part was the TBC story, or rather lack thereof, it was a hot mess and I think Blizzard realized they wasted the potential of the character so they decided to bring it back and rewrite the story.
Arthas is a tragic character - he became the very thing he wanted to defend against. Illidan actually succeeds in the end. And the price for it is fitting. Tragic would be if the only way of stopping Buning Legion was through sacrificing Tyrande or Azeroth, the 2 beings he wanted to save.
So... is everyone just pretending he didnt murder a naru infront of everyone now?
i feel people should be more... angry at him for doing that then they are...
Pretty sure he's popular because whenever you remeber Warcraft 3 you either remember Illidan or Arthas. everyone else was like "oh that tauren guy who looked like a Tree with bad textures" or "Mounted Orc". Of course there are exceptions, but i know a plenty of people who doesn't know warcraft that well but they do know Illidan and Arthas.
and i dare say Arthas is more popular, but he just isn't around enough to be that poplar.
Signature was infraaaaaaaaaacted. Need a new one!
He defended himself.. so.. while technically murder noone can blame him for it. Imagine a jehovah's witness just entering your house and forcing you to listen to their nonsense - while you might not kill them you will definitely show them the door, forcefully if you must. If that same jehovah's witness came at you with a knife..
And what's to be angry about? What is a Naaru to me? or anyone not connected to the light.. Hell even Velen said what she did was rather odd.
Good guy walks up to world ending threat, bashes with hammer and becomes hero. - Typical warcraft narrative
Morally ambiguous character resorts to desperate, despicable measures to defeat world ending threat. - Illidan narrative
Illidan's story is relatively unique and more logical as well. Villains become meaningless when they accomplish nothing and just roll ever when 20 random people walk up to them.