There are quite a few. The guy in Bruma (and indirectly his bodygaurd) a few people in the Whodoneit quest, Adamus Philida, and the Draconis family.
Out of curiosity, who asked for Rufio's and Baenlin's deaths ?
I have to agree, the DB questlines are great and deliver the feeling of being truly evil without any emotions, dedicated to the Nightmother.
Spoiler I heard about this fast, fast horse from my sister.
I couldn't get myself to kill somebody innocent to get the inital quest going, but I really, really wanted a fast horse and was short on money for a long time. Then I met this adventurer in a lonely cave, he ran into bandits, well, I did the final shot and voil?, Lucien visits me.
It took me a long time, until I finally made it to that tavern and murdered Rufio in his sleep. It was too easy. My character being sneaky and stuff really loved those questrewards, just like a drug. When I started to get my orders from the Argonian, I wouldn't ask questions anymore. Just go out, kill target, get payment, go for the next one.
Then Adamus retired. I didn't ask questions, I didn't care, all I was excited about was the rose. How could I manage to hit him with that arrow, without killing his guard. Got disgusted by cutting off his finger, but bonus here I come.
The next step, the purification. I hesitated, I liked some of my brothers and sisters too much. Luckily I was given some poisoned apples, so the task of getting them to eat that apples won over any friendly feelings I have had towards them.
Then the betrayment, something was wrong, terribly wrong. The paper changed, the style of writing changed and when I killed that Khajiit in his house, the discovery, he had the Black Hand's clothes in a barrel. I screamed, what did I do ? At this point, I wanted to stop, but couldn't.
Ah, Lucien, we can still turn this, we can get the traitor and save the rest of the Black Hand. The disappointment, when I came too late.
I couldn't really stop the action and stayed silent, like I did all the time before.
When I as the new listener stood in the purified sanctuary and that new Dark Brother murderer talked to me almost like worshipping me, I felt strange. What is this brotherhood consisting of 3 members ? I felt everything was wrong in the end, we just killed each other.
Took a ride with shadowmere outside. It was night, the moon shining and I realized what I've become, an evil being, death written all over. Suddenly so lonely. No friends, no passion, nobody to rely on. The guards calling me dirt. The people in the streets, getting nervous, not telling me anything anymore. My only companion an evil horse with red eyes. Is this Sithis ?
Next day I see this Black Horse Courier. The Dark Brotherhood murdered Adamus, this noble man, who served for law and order his entire life and only wanted to have a peaceful retirement. That was too much.
I really stepped back a bit, watched my monitor and thought, wow, this questline is really well-made.
I don't like my own character anymore.
She is helping everyone and everybody now, trying to save as much innocent life as possible while praying to the Nine everyday. They just don't answer...