I wonder if Sariel was similar to Kurono except she had developed an affinity to holy/white magic rather than dark/shadow magic and she never managed to free herself. Which might be why she sort of wishes him well and she might be a bit envious of him as well because I doubt that she has anything to cherish or protect where she's at. So, she'll obey orders and won't go against them exactly but if her orders don't tell her to do something that she should if she was wholeheartedly invested in the Cross Church cause but she doesn't want to; then she won't (ie letting Kurono go and hoping that he'll run into the capture squad in order to wipe them out.)