We've been through the age of the TV anti-hero, and seen many a good guy break bad. And while it's fascinating when a supposedly heroic character turns to the dark side, whether it's a brief flirtation or going past the point of no return, it's at least equally as compelling, if not even more so, when a villain goes in the opposite direction.
To have someone presented as bad, or even totally evil, and then take them on a journey that leads them to a point of redemption is, when done right, a completely thrilling and satisfying character arc, even if at the outset you might think seeing them as anything other than the bad guy is an impossible proposition.
Sometimes it might work because the actor playing them just has so much charisma it's difficult not to root for them anyway, other times because the work is just so well written, or perfectly fits the plot and takes the show in interesting new directions.
As they almost say, you either die a villain or live long enough to see yourself become the hero. Contains spoilers for all the shows and characters featured.