Of course Fai betrayed Ashura- it's almost impossible to call sealing your king anything BUT betrayal, even if the king was evil (and we don't actually know that Ashura was evil). I think most people think that Fai has something on his conscience, though, even besides betraying Ashura. It would be far too simple for Fai to be completely innocent. After all, not even Subaru was completely innocent and pure.
As for the Seishirou bit... I assume by his act you mean when he hid that he was the Sakurazukamori in Tokyo Babylon? Personally, I think Seishirou, in his own unbelievably effed up way actually did love Subaru. That's my interpretation of the infamous Rainbow Bridge scene. Oh god, I have to go read it now. It makes me cry... Clamp knows how to come up with the most messed-up, sick love stories EVER. And this post is completely off-topic, and makes no sense to those who haven't read TB or X (myself, I just finished reading X, or at least, all of X that's out. Like, about five minutes ago I haven't been doing much else today except reading X). I hope nobody minds. I'll try to connect it back to Tsubasa somehow... Oh, yeah. So maybe the whole Ashura/Fai thing might end up being somewhat like Seishirou/Subaru. That would be fascinating.
