I dont't think he was that much retconned. His story inside WOW made some of his motivations more clear. But yea he was pretty reckless.
Him bodying Xe'ra then Sargeras in the same expansion was cool as fuck to see.
I felt he was a completely new character playing Legion and reading about TBC.
Especially Legion.
Xe'ra is there giving him a freebie PU and he rejects it?
Not war3 Illidan.
War3 Illidan would have played along on the Light Bondage game all patch long.
Malfurion "you made pacts with demons to get your powers. you're a menace to us all!"
Illidan "I'm being treated unfairly"
Also Illidan 5 minutes after Tyrande releases him "so, arthas, you telling me there's this warlock artifact that will grant me these awesome demonic powers? let's fucking gooooooooooooooooooooo"