Originally Posted by
Qualia
This is actually wrong.
The Legion, and thus Azshara, already knew about the Dragon Soul / Demon Soul before Illidan pretended to join their side. Illidan just found a way to track Deathwing in order to attempt to steal the disc. Moreover, while you can say that he was the reason Sargeras almost came out of the portal, you also need to keep in mind that he was the reason why the Legion was banished and the NE alliance even won at all. Sargeras would eventually come to Azeroth sooner or later during WoTA - it's just a matter of time for the Legion to gather enough power, yet the Azeroth resistance force s(other than the moment with Deathwing using the Dragon Soul) are weaker than the Legion with Archimonde as the commander. In a way, you can say that Illidan merely hasten the inevitable for his own plan.
Moreover, Malfurion didn't even think of what to use the disc for, he was just trying to steal it so Deathwing wouldn't be able to use it against Dragons, allowing them to help the NE force. The disc would have been wasted - only acted as a very powerful weapon in Malfurion's hands. The spell to reverse the working of the portal were originally carried out by Illidan, Malfurion just completed it when Illidan were having problem. While we can give Malfurion credits for this, without Illidan, the idea might not even occurred in Malfurion's mind. That's why he wasn't neither imprisoned nor punished for his actions during WoTA after the war.