I have a whole list:
Elenwen - She is the one who ordered the justiciars to kill the adoptive Nord parents of my Thalmor Slayer Khajiit. I just enjoyed to see her scream like a witch when I soultrapped her.
Arcano - The queen b*%$# of Mundus learned the hardest way that one who is lust for power fails to see the tragic ending. Like Elenwen, I soutrapped him.
Telrav - Lures people to Nilheim, where his bandit buddies will slaughter you and take your stuff, but I did different. I killed his men first and then I loved his reaction when he lured me to the trap and saw his friends dead.
Alain Dufont - Arrogant bastard stole a family warhammer and basically shamed an entire family with it
Hemming ans Sibbi Black-Briar - Sadly, Hemming's essential but both are a complete waste of human life. Also Hemming might be an idiot considering the fact that despite saving all of Skyrim, he still thinks your skills in combat are lackluster.
Jaree-Ra - Had the audacity to get angry at you for killing his sister and all the Blackblood Marauders when HE'S the one who tried to have you killed.
Atar - Silver-Blood thug who tried to oust Ainethach from the silver mine in Karthwasten. Murdered him even after intimidating him into leaving.
Thonar Silver-Blood - If you did the quest "The Forsworn Conspiracy" you'll know that he basically turned Madanach into his political weapon. I soutrapped him and used his soul to enchant a silver necklace with the silver I mined from Cidhna Mine.
Grelod - If you talk to one of the kids before killing her, I think Runa Fair-Shield, she'll tell you about the room with shackles in it but in a way that sounds like she got molested. I soutrapped her, enchanted a wood sword with her soul and gave it to Runa.
Brelyna - She clearly didn't pay attention when Toldfir said that magic must be studied with caution or else disasters are inevitable. Talking to Phinis reveals that some students tried some experimental magics and they never returned and yet she asks you to be her test subject. Of course I asked her later to be my test subject: try to endure a barrage of Ignite followed by a soltrap.
J'Zargo - I was REALLY pissed off when I told him the "twist" he mentioned about his scrolls is an explosion and instead of appologizing, he said "at leat you are here and not in pieces". He had the same fate of Brelyna
Calixto Corrirum - Creepy serial killer who tried to resurrect his sister. You'll find proof of this in his journal if you take his key and use it to open his unpickable chest in his little museum in a shelf space directly above the doorway where you walk in.
Malyn Veran - Insane wizard who achieved a limited form of immortality. Soul trapped him for the irony.
Sild the Warlock - Sadistic wizard. Likes trapping you in a cage and then burning you to death. Gave him no mercy.
Naris the Wicked - Same as above, but at least he isn't as stupid as to try and kill you.
Mogrul - Words cannot even describe how much I hate this pig face. At least he's smart enough to back off if you sucessfully intimidate him but I prefer to murder him and send his soul to a better place that Ashpit: Soul Cairn.
Degaine - You offer him a gold piece and he thanks you saying "thank you now get lost. And bring more gold next time". Soultrapped him in order to be more polite.
Bolund - Racism is something I don't tolerate even from my own parents, so I soultrapped him hoping in Soul Cairn he can be "much in the way o' conversation" with other races.
The Silver Hand in general - "He who fights monsters should see to it that he himself does not become a monster".
The Imperial captain who tried to have you executed without a good reason - B#$%.
The adventurer who taunts you and calls you "milk drinker" - the idiocy of their belligerent statements is... jarring.
Those three "Soldiers" who basically ask you hand over 100 gold - They should be happy all I did was kill them.
The leader of Faldar's Tooth - "So you're the one who's been killing all my dogs? You'll pay for that!". Well, if you mean all the bandits I killed, then yeah I did.
Vampires in general are the most satisfying kills: "The only blood you'll be tasting is your own, nightspawn". Thalmor and general warlocks come close
The parents of annoying children - Since console players can't kill children, I make them suffer by killing their parents instead.