I would say that stealing the phylactery proves that, at the very least, he is committed to the possibility that destroying Xykon might become his #1 priority.

If someone happens to destroy Xykon (even if Redcloak isn't actively seeking it at the time) then now he absolutely has to finish the job. If he let Xykon come back again then Xykon would immediately know about the theft of the phylactery and all bets would be off.

If finishing the plan with Xykon's help was really the top priority then I don't think he'd jeopardize that.