So I've gotten a bit further with the Thieves Guild questline, I'm enjoying it overall but honestly I wish the whole bit with the Nightengales was different.
Click here to see the hidden message (It might contain spoilers)
in order to be inducted into the Nightengales you have to pledge your loyalty to a Daedric prince in both this life and the next. I'm not exactly fond of pledging loyalty to any Daedric princes and I wish they had an option to refuse induction but still permit helping the Thieves Guild sort out their problems. The dialogue also made it feel like Karliah had suckered you and the other thief into agreeing to be "inducted" when you were in reality both just being used by Karliah as a sort of payment so she could get back in the good graces of the Daedra.