Kso...
Ukitake being the "evil" one? I'm not buying it. I can imagine him being part of some "plan" or "organization" (with kinda good intentions), created to "deal" with sub-soulreapers and their powers and the way they can affect SS (or maybe using them to change smth in SS), but the "evil" one... Also I can not see other captains, who've become Ichigo's friends, being "so" blind about Jyuushirou. Okay, Aizen was Aizen, after all - and now you suggest Ukitake outsmarted everyone else by 'himself'? Especially with all of those mysteries, like Rukia's erased memory and Hitsugaya's glares? If Ukitake is really behind some great plan - he's definitely not alone in it.
If he's not, than what do we have later? SS fighting against each other (Ichigo's side vs Ukitake's side)? No, it'd be too boring.
Come on, Kubo, get a grip, it would be just so lame and it'd spoil one of your best characters! Such turn of events is SO below you.
I can see Jyuushirou having some shadows. No one is perfect and saint. And he is too old to live without some secrets of himself. He is also a captain - and that brings responsibility and the duties you can't ignore - even if you have to lie or withhold the truth sometimes. But - ARGH - him falling so low is just BEYOND my imagination.
I think even this small piece of truth leads us closer to the King's arc. I can so imagine Ukitake being part of a plan to "overthrow" the mysterious King or smth like that. But, truly,a PART only. This way the talk with Aizen, when he was leaving SS, makes sense. Too arrogant as in 'aren't you believing too much in your own abilities to do it?"
Anyway, Ukitake, I'm still loving you as long as Shunsui will be by your side, you poor old sick man. Ahaha. I'll believe in you till the end. And if you're really evil, ah... to hell with canon it'd be.