When I think about it, I don't think I've actually played a TES character who has been properly, properly evil at any point, really. Not sure why, but there we go. I think the closest I ever came was with a Dark Elf assassin/thief character I played, who I did the entirety of the Dark Brotherhood and Thieve's Guild questline with; with her, however, she generally didn't do it for any sense of the thrill of killing or anything like that, but instead because it was something that she was good at and it paid well. She would go to the location, find the target and eliminate them, just like that, driven by a sort of cold, dispassionate professionalism; so long as she didn't form any kind of attachment to the target she simply didn't care for them, and their deaths were simply for the sake of profit. On one hand, it was heartless to the point of being sociopathic, but on the other it was always as clean and quiet as possible; she actually found it rather difficult to do the whole 'cleanse the sanctuary' thing because she'd got to know the rest of the Brotherhood members there, but when she found out she'd accidentally wiped out half of her organisation she didn't feel that bad because she never knew those targets and felt no bond towards them.
That was probably the closest I came to being overtly evil in a TES game, but it's kind of tricky to be a proper, proper bad guy in those games; aside from the Dark Brotherhood, there isn't really an overtly 'evil' faction about. In the Fallout games, on the other hand, I find it much easier to be a puppy-kicking, town-burning, raider-fistbumping, complete monster who makes babies cry just by looking at them. I think it's because Fallout lets you style your character's hair into a mohawk, for as we all know, in post-apocalyptic settings people with mohawks are evil!
