I've read that line and while its true that its sounds like Cas will be killed I don't think its gonna be in the premier episode. They turn Castiel into a super charged angel only to be killed off 30 minutes later No.
Castiel is Castiel. He'll try to do goodbut the road to hell is paved in them.
I think season 7 will play out in this manner. It will start off right where we left off of course Sam and Dean ain't gonna bow down and while they care for Cas I doubt Deans gonna say I love you to him (maybe there are slash fans who may like that but I doubt it.) The boys will get out of it and Cas will leave them alone for awhile (at least thefew hours he needs todeals with Raphs followers)
Cas declarations make me think that he will try a more hands on approachin his management of the planet. His final goal is to bring heaven to Earth. He'll try to right asmuch wrong as he can
1.meaning he'll probably organized demon killing units and hunt down the Lucifer loyalists and Crowley as well.
2. have angels actively "helping" peopleand smiting things.
3. Probably create a paradise falls where people are shiny happy and provided for.
Of course it all sounds good but Cas may get too effiecient. You know that kind of super righteousness that no one can measure up too. So he ends up killing more people. Which the boys will try and prevent where they can.
IN the Finale Cas will be frustrated that he'll do a "Noah's ark" well Dean's ark or Singer's autowhile he waste everything else.OR It may turnout that holding the souls of Purgatory is slowly poisoning Cas. First his drunkness for power as we've seen and he'll blowS up.
Maybe there will be one episode like the man who would be king that Cas will show that while he is doing his "good" deeds he was watching Sam and Dean and were secretly hoping they can stop him from going over the deep end.
THis is more general and mostly speculation on who will show up:
1. Crowley's probably thinking up a way to suck up Cas is supply.
2.Meg would probably try to run and hide to the Winchesters
3. Oh if Cas really is or close to what he claims he is I'm betting Dean will go to Death for help. (Of course he probablywon't interfere with Cas but I think it will be great to seehim in season 7). Hey maybe Athropos (or the other fates) will show up.
4. Mommy May show up again since her supply is tapped