I also agree that Xykon's offer was genuine. Xykon is looking for something that is worth... err... unliving for.
A LG adversary is just what he needs. Xykon is also looking for great fights (as that makes great villains) and he knew Roy could not offer him that, so it would have been "boring" to kill him at this point.

Xykon has been looking for a purpose since we met him in SoD and he never really found it. He thought he found it in what Redcloak suggested, but that so far takes decades (and has seen some major setbacks). So an alternative isn't too bad.

Xykon does not want to get destroyed and likes to unlive, he'll cling to that with everything he's got but I think he'd also be ok with going down in a truly epic battle (in this regard he's living what Tarquin claimed to hope from Elan).