My theory (which would require them to actually care), is that it's not Illidan. It's Gul'dan. Everything starts to make sense:

Figure this - Illidan consumed the Skull of Gul'dan, which is how he could get to Outland when no one else could (ignore the Dark Portal randomly opening, please). When Arthas stabbed him, he *did* kill Illidan and took his soul... but Illidan wasn't alone, soul-wise, in his body. So, when "Illidan" woke up, he was actually Gul'dan. It all starts to make sense: sucking up to the Fel Horde and making them stronger by giving them Magtheridon; moving into the Black Temple, Gul'dan's seat of power; enslaving the Draenei, whom Gul'dan hated; marginalizing the blood elves, whom an Old Horde warrior would despise; you can throw in homophobia if you want, if you assume Kael and Illidan were involved pre-Northrend, and Gul'dan was disgusted by it.

Kael gets to watch the person he loved and trusted, the person he nearly died defending, the person he vowed to follow into the heart of hell itself, is suddenly turning his back on everything he's ever done and ever cared for, and he loses it. His people are now addicted to fel magic, which is like trading in a nicotine addiction for a heroin addiction, he can never go back, his closest friend (Jaina) probably hates him because he said stupid things to her before he left... so he turns to the demons, and Kil'jaeden is all too willing to use him up and burn him out. So goes Kael's slide towards insanity.

Also? It makes sense that "Illidan" feels that he won against Arthas... because Gul'dan won when Arthas killed Illidan. New body, tons of power, the ability to go 'home' and take his rightful place amongst the Burning Legion. If it weren't for those meddling kids...

(I may have thought about this too much. I also may imagine Kael haunting Jaina, miserable and trying to tell her how sorry he is, but he can't because she has zero spirit-sense.)