Illidan Stormrage: Illidan is a strange one. He's never really had much interaction with him in canon beyond his impedance of Arthas reaching the Frozen Throne. However, despite past wrongs against his master, Kel'Thuzad recognizes Illidan for what he is: much like himself, a being who lusted for power (any and all power) and was cast out by society for it. He also recognizes that Illidan cares only for self-preservation, and worries not about the greater good.

Would he enjoy having Illidan as an ally? Yes and no. Having such a potent beacon of magic besides himself is extremely beneficial to his aims. His attitude kind of rubs Kel'Thuzad the wrong way and the fact that Arthas doesn't seem to like him really doesn't help matters. But the fact that he is sided with the other elves right now doesn't sit right, and he despite what he says, he does perceive him as at least a minor threat.