I don't know, she seemed to be a professional then, and a professional now. She's just a professional who owes a serious mortal debt to someone else. And hey, he wants to risk her life fighting some Imperial remnants, its not like she has moral objections to either risking her life or killing Imperials.
I mean, some of this is just me liking Ming-Na Wen, but a lot of it is thematic interpolation. Toro Calican was set up as the complete unlikable scum kind of amoral mercenary, and he literally shot her. While there is no physical rule that prevents them from being equally scummy mutual betrayers it works better as a story if you actually are supposed to hate Toro for this. . . and you hate him more if Fennec is more respectable than him. Thus, my interpretation: Toro is a treacherous backstabber who will shoot unarmed prisoners and hold innocent bystanders hostage, while Fennec is a professional who keeps her promises and would be slightly offended if you implied she needed to shoot random bystanders.