SPOILERS FROM THE THIEVES GUILD QUEST LINE just in case someone hasn't done the quest yet
I'll actually have to say Karliah, and this is why:
She puts an arrow in your chest...Made a "split second decision" my a**! Mercer was right there! I'm sure you could have shot him if instead if you really wanted to. Even worse is the fact that she expects you to be grateful.....for getting shot! Because after all, that poison did take her a yeeeear to perfect.
She lures you into nightingale hall without ever telling you why, even after getting the nightingale armor on, she just keeps leading you deeper into the cave only to reveal that the purpose of you being there, is to basically give your soul to Nocturnal.
And that has nothing to do with Mercer, we could have beaten him without having to sell our souls, it is my firm belief that Karliah did all that just to regain Nocturnals favor. The whole "but Mercer is a nightingale and we can't defeat him without giving up our souls" is just total bs. She wanted to get back on Nocturnals good side so she sold you out.
I know a lot of people like Karliah but I'm just not feeling her. The whole nightingale thing just ruined the TG quest line for me.