I get a perverse amount of pleasure from killing Cicero.
I typically regret the lack of choice in Skyrim quests (you can't side with Mercer to work against Nocturnal because PLOT, you can't team up with Arcano or join the Thalmor because PLOT, you can't konk the heads of Ulfric and Tulius together and say "You numbskulls, you're aiding the dragons with your little pissing contest!"... you get the idea).
I hate having to see the common folk die from randomness because it's quite grim. Like, Sven is just your everday unemployed artist, busking for a living with his lute, until this dragon swoops down from the sky and murders him in front of his mother.
I hate pretty much everyone (who's not a civilian... with a few exceptions).
Thonar Silver-Blood the manipulative bastard,
Beem-Ja (Ironbind Barrow aka the place with the battleaxe of firery souls) because he betrays you.
Commander Maro for destroying your Family
Telrav because the bastard tricks you into helping him cross a bridge and then getting his guys to kill you (you can kill his men first and he'll be pissed at you... which pisses me off even more for some reason)
The thieves who try to rob you on the side of the road ("I don't have time for this")
Mogrul because he always has a stick up his ass
All the Forsworn
That one bandit in Ravenscar Hollow who begs you to let him out of a cage before the Hagravens kill him, and then after you kill the Hagravens, he betrays you,
Taron Dreth because he stole Katria's (Lost to the Ages) research and screwed her over.
The three bandits dressing as Imperials.
Idolaf Battle-Born (Stuck-up asshole who taunts the Stormcloak-supporting, NON-RACIST Grey-Manes)
Degaine (Any beggar who insults you should just die already)
Bolund (Racist prick)
Rolff Stone-Fist (Very racist prick. Also essential)
Sild the Wicked (Rannveig's Fast) because he's a sadistic cunt who tortures people with fire magic
I am not a huge fan of Thonar Silver-Blood, I hated how he was using native Reachmen to do his dirty work. I found it gratifying to watch him die by Forsworn hands and then raised by another.
On the other hand, I wish Skjor didn't die, he never deserved to die the way he did. He was perhaps one of the smartest Circle members around, and seeing that he was killed by foolishness was very saddening.
The worst for me is that if you choose to kill Nilsine Shatter-Shield during Mourning Never Comes, then her mother, Tova Shatter-Shield, will kill herself out of grief. Nilsine's sister Friga had been murdered by the Windhelm Butcher not long before the beginning of the game, and if Nilsine dies too, Tova has nothing left to live for. This note cane be found next to her body.
What makes this truly awful is that it's totally dependent upon the player's choice of action during that quest. For a simple little ring, you kill one woman and consign another to her death. And it's 100% optional; you can still complete the quest without killing Nilsine, as she's not the primary target.
I reserve a special burning hatred for Nazeem. Heimskr also, and on my very first playthrough, as soon as I walked into Whiterun, I heard him shouting away. I knew that such a disturber of the peace couldn't be tolerated by my eardrums, so up came my bow and down went his body :D. Somehow I even managed to avoid a bounty, even if I didn't know what that was at the time. Mjoll I think is pretty cool, and Delvin is awesome, especially his voice. I love his reactions when you tell him you're in the DB lol.