Ilidan... He's not evil just emoFollow

Illidan is power-hungry and addicted to magic. In pretty much every case where he had a chance to increase his power, he did so, regardless of the source of that power.

War of the Ancients
Illidan betrayed the Night Elves by trying to give Sargeras the Demon Soul and allow him to strengthen the portal so he could enter Azeroth. (evil)

For reference: The Well of Eternity and its use (misuse, whatever), attracted the Burning Legion to Azeroth, corrupted the Highborne who became addicted to magic and allied with the Legion, and indirectly led to the creation of the Satyr and the Naga. Its destruction caused the Sundering, blasting Azeroth into two continents, and iirc creating the Maelstrom.

Illidan created a Second Well of Eternity after the Sundering (ambiguous, but probably a sign of madness/magical addiction, not necessarily evil). He was only jailed, not killed, after this act because of the intervention of his brother.

WC3
Illidan drank from the Skull of Gul'dan to give himself more power, and became a half-demon (having been told about the skull and its history by Arthas) (evil). Kil'jaeden visited Illidan, who enlisted to help the Legion destroy the Lich King in return for more power (evil). He then butchered a port town for ships with ~no remorse (evil), went to the Tomb of Sargeras (like Gul'dan) and got the Eye of Sargeras (again, a an evil item that gave him more power). He used this power to try to break apart a whole continent (Malfurion stopped him because he sensed that Illidan's actions were a danger to the world) (possibly just crazy, but as it was in the service of demons, I'll call it evil).

DESPITE ALL OF THIS, Illidan was allowed to leave with a promise that he not threaten the Night Elves again.

Illidan then cleared demons from Outland so he could stay out of Kil'jaeden's grasp. This was not a "good" action; he was scared of retribution for failing KJ. He then got *****-slapped by KJ and got worked by Arthas.

WoW
After defeating Mag, he did not restore the Black Temple as a place of worship for the Draenei (Karabor), but declared himself Lord of Outland and uses a former holy site as a fortress (evil). Instead of killing Magtherion, he imprisoned him and used his blood to create new Fel Orcs (evil). He keeps demons as lieutenants (evil). He attacked Shattrath city with Blood Elves (evil).

Sorry, maybe he's just completely ******* crazy and not evil. Still, you've got a crazy, power-hungry, vicious half-demon who is using his armies of Fel Orcs and blood elves to slaughter innocent victims who might oppose his rise to power. I think we can safely say that if he consolidated power, he would rule Outland as a tyrant. Oh, wait ... that would probably qualify as evil.

Of course, some people just squirm at the idea of calling anyone evil. Either you're a complete relativist (in which case, stay the hell out of this discussion, since you don't believe ANYTHING is evil) or you've got some cartoonish 8-year-old definition of evil where there are never shades of gray. Sorry.