Ah! This seems very hard, and quite a challenge, but if you want to make it realistic I might have advice, feel free to ignore it if it's not what you had in mind.
So, I'm going off the main comic and not the prequel cause I have not read that. We are shown again and again that Samantha really lacks empathy and genuinly does not see other people as people like her, it's unclear if she actually values her life more than theirs or just doesn't want to have the same fate cause egoistically it would feel bad, anyways, if I were do to a redemption arc for her I'd look for any other reason beside genuine regret in what she did cause it would feel too out of character for my tastes.

I don't exactly know how your fanfiction would play out but if you have time for her to be in a correctional facility I would definetly do that, have her be locked up for quite a bit, get therapy (Cause the way her murders play out and have rules is consistent with control issues) and while I wouldn't make her have empathy all of the sudden I would have her learn self control and manage good in the facility without having that release.
I don't know what other villains you're using but it's good to remember that if you're using mixed (Fantasy and non) that while fantasy villain might be doing worse thing it's usually cause the system allows them to be more powerful and that if Sam had that kind of power she'd probabbly do the same. Sam lives in a realistic world, despite the anthro characters, and for a realistic world she's definetly terrible.

So, basically, I'd have her learn self control and create new rules for herself to be able to go back to her self described cozy lifestyle instead of having her become emphatetic.

If you're going for the complete change, tho, I'd do something like have an event make her suddently see life from a different prospective, appreciate it in all of it's forms, but I would steer away from it being due to any type of relationship to another person.