It's why Cassiel's cool. He could use his great powers to all manner of things, yes, but he doesn't, to drive his point that the Grigori shouldn't rely on the God's at all, even for their own lives. Self-determination at it's best: it's the same reason why he doesn't grant the power to heal plagues but instead leaves the Medicos to trial and error, why he didn't use his own powers to make the Age of Ice so much less miserable, why he didn't use his powers in the God War or the Age of Dragons to protect the Grigori. Whatever they do, he's pushing them to do it without divine help, but instead entirely on their own. It's a matter of principle that's more important to him than life.
Why he surrendered himself so easily, I believe he made Auric agree that the Grigori people would be left alive in return for his cooperation.
As for the Bannor, it's not that they've become evil, but that the Order heirarchy became corrupted. Fewer and fewer people who genuinely worshipped Junil and their hearts were there, and those who did were increasingly persecuted by those who didn't. Those same priests who didn't were the only ones who would enter the halls of Sabbathiel, the Bannor leader, and so eventually they just issued their own orders in his name, eventually driving him to leave them (and no one knew the difference).