Xykon has a few basic characteristics:
1) He sacrifices his followers routinely.
2) He's become powerful enough that he rarely encounters a credible threat to his existence.
3) He has a really bad memory.
Considering #2, it's a question of whether he's overconfident or not. As a lich with access to 9th level spells and a phylactery currently of whereabouts uknown (if it's the crown, then clearly he doesn't need to be near a phylactery to revive...which I thought was counter to the rules, but I can't recall and the SRD is unclear).
Let's face facts, when you're effectively immortal, you're going to get bored and you're going to get overconfident. Without knowing WHY Xykon wants the gates (and can assume he has a plan for them), we can't really gauge his reasoning.
Oh, and I like Roy a lot. He's a LG, fairly smart fighter and the only thing that keeps the ragtag bunch that is the OotS functioning as a group.