Hmm, off the top of my head I can't think of any follower that is more morally questionable than Eola, the creepy cannibal woman that you can recruit upon completing the "Taste of Death" daedric quest and become Namira's champion. She's pretty messed up. But she's also pretty difficult to obtain - you need to go through a fairly long (though not overly difficult) daedric quest that will test your own morals as well. So if you want a description of the most morally corrupt follower in the game, that's my opinion.
However, for this quest (done it twice) I've picked up Benor. He's really easy to get (beat him in a brawl in Morthal) and is fairly slow-witted and annoying IMO, plus he's also a fairly incosequential loss in the grand scheme of your Skyrim adventures. However, he could be termed "innocent." Therefore, if you're still set on sacrificing the worst follower, definitely go with Eola. As a bonus, she's beloved by Namira, so Boethiath would probably be pleased by her death as well......
