Illidan was like hey check it out i staved off the demon invasion by using their power against them. And then his brother Malfurion was like, haha fuck you 50,000 years of solitary confinement. And then 15,000 years later, Tyrande just goes like.. hmm. Well the bad guy of WC3 is a demon... and Illidan.. hunts demons... hmmmm.....
Then Malfurion is like nah fuck you Illidan go away.