Relationship between Illidan and Malfurion

The relationship between the Stormrage brothers is noticeably different in Warcraft III/The Frozen Throne and in the War of the Ancients Trilogy. 

By way of comparison, in the novels written by Knaak, Malfurion is more tolerant and forgiving of his brother, who is portrayed as often selfish, reckless, out of control and not completely sane. Rather than becoming part of the "lynch mob" who want Illidan summarily executed, Malfurion speaks on his behalf, endorsing his knowledge of the Burning Legion as vital tactical information... giving an undeniable reason to bury Illidan rather than kill him. 

When we first were introduced to Malfurion and Illidan in Warcraft III, Malfurion objected fiercely to the idea of freeing Illidan, in the belief that his crimes were unforgivable and that he should stay locked up. When they were reunited Illidan was bitter about how he had been punished and Malfurion was disgusted to be in the same room as Illidan, still nursing a grudge over Illidan's betrayal. Illidan, who seemed to be the more forgiving one at the time, was eager to prove to his brother that the demons had no hold over him. Later, after Illidan used the powers of the Skull of Gul'dan for himself and took on a demonic appearance, Malfurion demanded to know where his brother was, implying he still had some affection for him. After Illidan revealed that he was the demon, Malfurion, rather than killing him simply banished him. Obviously, Malfurion knew Maiev Shadowsong would want Illidan imprisoned but still chose not to act. Rather than object, Illidan simply walked away. 

Illidan was next seen in Warcraft III: Frozen Throne. After the two worked together to save Tyrande, Illidan seemed to have forgiven Malfurion for disrupting his attempt to destroy the Frozen Throne, even though it would leave him at Kil'jaeden's mercy. Malfurion's last words to Illidan included an ominous threat about what would happen to him if he ever did anything against the Kaldorei again. This too, Illidan accepted without objection. Illidan admitted that he had known only "ages of hate" for his brother, but for his part he wished it to end. This is perhaps the closest thing to a peace that the brothers have achieved. Illidan then departed telling Malfurion he doubted they would ever see each other again. 

In World of Warcraft, Malfurion's words indicate that he does not wish to fight his brother, and he says that if he is forced to, things will not be as they were at the Well at Zin-Azshari. 
Current state 

From Malfurion's conversation with Remulos: 
Malfurion Stormrage says: Cenarius fights at my side. Illidan sits atop his throne in Outland – brooding. I'm afraid that the loss to Arthas proved to be his breaking point. Madness has embraced him, Remulos. He replays the events in his mind a thousand times per day, but in his mind, he is the victor and Arthas is utterly defeated. He is too far gone, old friend. I fear that the time may soon come that our bond is tested and it will not be as it was at the Well in Zin-Azshari. 

The exact meaning and implications of this are not known. It is however likely that Malfurion fears that the bonds between him and his brother Illidan will be tested in the near future. For 10,000 years ago, during the War of the Ancients, the two brothers were forced to work together to throw back the Burning Legion at The Sundering, which is when they chose their separate paths. Now, with Illidan crazy and changed in many ways, such cooperation may prove to be very difficult.