We know that Xykon claims to be fine with his undead condition. He was not at all reluctant to change, and he openly claims it's a logical part of his life plan. "Anything to avoid the Big Fire Below
But Xykon-the-living never had much of a life plan - like Belkar, he lived very much in the moment. I doubt if he ever gave much thought to the BFB. It was Redcloak who raised his sights to world domination in the first place and set him to thinking long-term; it was Redcloak who guided him into the fix that made it necessary to make him a lich, and it was Redcloak who performed the actual procedure. In a sense, Redcloak created
Xykon as we know him.
Maybe Xykon holds a very special core of hatred for RC for that. And maybe he figures he owes it to RC to make sure that he (RC), of all people, gets to experience personally the full fallout of Xykon's inhumanity.