If Minthara can be redeemed, I suspect it will play out the same way. I hope she stays pure evil whether she joins the party or not. I would love to see the final narrative story explaining how she took back her house and is now a matron or something like that.
Well I personally hope not, it would be super weird trying to ''redeem'' someone after you slaughter some kids to get on their good side. I like her as the character she currently is and don't want her to change.
Also evil is in the eyes of the beholder, you can be evil to some while being good to others, I don't want to be on Mintharas good side, but that doesn't mean that she needs some sort of redemption.
What do you think character development is?
Characters ideals and personality being challenged over time and becoming more solid or going through change.