In Knights of the Eternal Throne, I found the way Bioware handled redemption to be very intriguing.
There are two main villains in the Zakuul saga: Arcann and Vaylin. Both are children of Valkorion and your enemies for most of the game. Arcann is Emperor of Zakuul first, until the player defeats him at the end of Knights of the Fallen Empire, at which point Vaylin becomes Empress.
In KotET, Arcann can be redeemed, brought back to the Light and becomes a companion for the player if the player desires. Vaylin cannot. However, Vaylin arguably is more worthy of being redeemed.
Here is Arcann's trailer:
And here is Vaylin's trailer:
Arcann chooses to become a villain. He makes his choice in anger and rage, but it's still his choice. Vaylin, on the other hand, is conditioned into villainy as a child. She doesn't really have a choice. Even though she probably commits greater evils.
KotET goes to significant length to lay this out for the player, including an excellent chapter on Nathema where it goes into detail about Vaylin's imprisonment on Nathema, and the experiments conducted on her there.
Yet in the end, Vaylin cannot be redeemed. This lends a small sense of unfairness to Arcann's redemption. He is perhaps less deserving than Vaylin, but gets a better end.
I'm not saying this is a negative for KotET. Quite the opposite. It was an excellent move on Bioware's part. It adds a touch of bittersweet-ness to the ending, makes it not quite so perfect and shiny. In fact, I even found myself slightly regretting redeeming Arcann because of that unfairness.