It's less about the act itself and more about that it's easier to kill somebody in a dark room where nobody will know than to take a slave through the entire city without having to face people trying to stop you and having to kill then, including, for example, the two guards literally at the entrance of the last floor.
The same thing goes for buying her. It's established that Karnak people hate Astyllians, plus her master is basically a pig, so even if Taliya had that much money on her somehow (a slave is pretty costly, and before the last time you go to Karnak, Taliya went there with the idea of staying just a day, and even if Karnak's inns are costly, a room for two for a night is worth less than a slave), her master would definitely want to maker them pay twice her actual price or more and you'd definitely not have that with you on a mission.