I understand, and I appreciate the help.

I thought it might be fun to brainstorm the ideas here.

Ah, sorry about that then. In that case, I'd be happy to help further, if I can.

I would say that if you're going to go with the, "she knows too much," coupled with the, "she's tired of killing," angle, I would suggest that there was a triggering event. Perhaps she saw something that shook her faith in the empire. Maybe she wavered on her last job and her target rewarded her for her mercy. Maybe after making her decision, she used her position to acquire an item or data that she could sell to ensure her retirement fund. Regardless of the reason, you'll find your story much more compelling if you can come up with a specific reason for why the empire is chasing her; it's unlikely the empire would waste resources hunting down every deserter, so she must be special.

If she does have some item/info, it would be good for her to hide this information from the PCs. Let the players wonder as to why such a huge force is coming after her (if the players don't figure this out on their own, as a GM, I usually make a point of telling the players when a show of force is unusually larger or smaller than would be expected). This could make for some interesting drama and some potential renegotiations once the PCs learn how valuable she truly is.

Not sure I like the idea as a whole. It might make a good adventure at a later stage, but for now it feels like the characters are second fiddle to the NPC and don't get to be themselves.

Maybe a different idea:

An Imperial defector - someone with compromising knowledge useful to an underworld patron such as a Black Sun figure or a Hutt - needs to be 'captured' by the pc's (pilot and hired gun). They bring him to underworld space/the outer rim where, in return for what he knows, he's offered sanctuary from the imperials that want him dead (ie not talking).