That surely deserve 10,000 years of solitary imprisonment in a room less than 9 steps in any direction, with magics used specifically to keep him from suiciding. And Illidan didn't led Varo'Then to capture Malfurion, he led Varo'Then to come and steal the disk from Deathwing - Malfurion just happened to do it first, so they took it over from it. In the end, all Illidan's actions turned out well - and with good intents - for the war, after all. It was Illidan who first thought of the idea of using the DS to banish all demons from Azeroth and Malfurion just capitalized on it.
(In comparison, instead of doing anything in form of taking revenge, Illidan either walked away from Malfurion, eager to prove that demons had no hold over him, or told Malfurion that he didn't care about their bad blood anymore and left for Outland)
He allied with the Legion to try to overcome them from within, as per both WoTA triology and "Illidan" book. It's true that there were selfish goals in that action, but there were good intentions, too. We can't just look at the selfish part and ignore the others. In the end, his actions turned out beneficial for the world.
Same for replicating the WoE. The Well didn't "nearly destroyed the world" by itself, Malfurion's spell did (although, it'd probably be the same if Illidan did it). He figured out that Arcane was a needed power to fight with the Legion who'd definitely one day return. The NE didn't agree back then, but as of WC3 and recent events, it turned out he was right.