You did it for a kiss. You loved Carles and swore fealty to him, but you loved him more. So when the choice came, you did not hesitate.
You weren't sure why, but something about him was different from the time you first saw him. He was strong and brave and handsome, and you thought that you would do anything to be with him, even betray your lord and become a Moor.
When he first saw you, he stopped and did not kill you but took you away. He took you away, and the two of you lay with each other at some place free of humans. When he asked you if you would do this again, you said you would stop at nothing to lie with him again.
You only ever lay with him once more, the day when he and his friends came to you to ask you to betray Carles. You readily agreed, and he kissed you upon your lips for it. When night fell, the two of you lay together again, and you thought you could be together with him forever after the Frankish army was defeated.
But Oliviers killed him, and you knew that Carles would kill you as well. You didn't regret your betrayal. Not for what it gave you in return.