Shiida won't turn out to be evil. We've already saw her with 6 Fingers. Her story evolved into finding good and happiness with Iruma around. If I have to guess, she will find herself in a situation that forces her to choose to betray Babyls for the sake of Iruma or something. Unfortunate, but I believe she will be happy after all that.
I'm not so confident about that. A big thing with the Six Fingers seems to be that they have some sort of twisted aspect, at least going by Atori, Baal, Kirio, and Ocho. Sof far, Shiida seems completely normal, if a little bit poorly socialized, and it's hard to see how she'd fit into the group like that.
Part of me suspects that she either runs of a blue/orange morality or she has some sort of gap in the way she sees the world that lets her perform, or at least be comfortable with, ultra-violence while still maintaining her innocent demeanor. If that's the case, I could see her acting against Iruma and not really perceiving it as a betrayal or genuinely sticking with him, but causing some other problems with her reactions.
