Curious

i think she first said that because she knew that the prince was a "villain" and she didn't want to commit "villainous" deeds with the fear that it could bring her into his downfall. however, now that they're close and will get closer i'm assuming she'll kill for the prince.

No, I believe Selena said it cause she knows the prince would order her to kill Latio in the future — just like how the prince ordered Celestine to kill Latio in the novel. And we all know the result of the assasination — Celestine getting killed. At that time, she was so absorbed in the fact that she had to avoid every possible link to the original characters to avoid getting executed. And now that she has to act like she was Celestine, the pressure of interacting with Prince Ignis scared her. She is scared of ending up like Celestine in the original book.

mc just meant she doesn't want to be turned into a assassin type escort

Thank you. I feel like many don't remember this part and it's pretty important since it distinguishes what she personally believes is or is not ok to do. Just to echo your words, yeah she told the prince no killing because she doesn't want to be tasked with attempting to assassinate Letio (and fail), but with the way she worded it to the prince, it just sounds like she doesn't want blood on her hands. So from the prince's perspective, she's like a pacifist.

With this note, hope she doesn't do a double-take and do something silly like adhering to that code. MC has a tendency to try and keep up appearances for the main story even though the story is not as relevant anymore since it's changed. We can see that she's more or less realized this already, but she's still struggling to keep everything under wraps.