Delphine. How DARE you ask me to
Spoiler kill Parthuunax? He's done more for me than you EVER did. I thought you were COOL, lady. Turns out you're just a big ol' dragon-racist. Pffft. Whatever.
Cicero, most definitely. Killing that little bastard felt AWESOME. I know you have the OPTION not to kill him, but MAN. It's almost irresistible, isn't it? He's clearly been specifically designed to be as wheedlingly annoying as possible. I hated that little knob so much that after he was dead, I dragged his body over to the brazier at the end of the room, and draqed him over it. And even though Nazir and Babette have moved into the Dawnstar Sanctuary, and there are currently prisoners chained to the wall in that room, he's still there...stripped nvde, and cookin' away. Seems fitting.
Belethor, and the elves from the Radiant Raiment. Look, I know being a merchant can't be ALL giggles and sunshine, but you lot have some of the svckiest attitudes imaginable. Oh, and also that Nord jerk who runs the shop in Falkreath. Lemme get this straight...I drop thousands of Septims with him, stocking up on stuff, and he still has the stones to threaten me if I try to steal anything while I'm on my way out the door? Y'know, I'm thinking I might actually try the trick
W1tcher mentioned, and rob Belethor's sorry behind blind when I get home.
But the #1 toolbox I wish I could kill? Ulfric. MAN, I hate that guy. I actually loathe him so much that I actually tried to send him to Sovngarde once. I just marched right up to his slouching, sneering carcass right on his pretentious throne in Windhelm, and introduced him to the business end of my Legendary, Fire-Enchanted, 270-damage Daedric Warhammer, "El Smasho the F**ker-Upper." After enough bashes to the melon to bring down an Elder Dragon, he just stood there scowling at me as the guards tried taking me out.
It may not have worked, and I had to reload the auto-save from when I walked in the door...but at least I felt a LITTLE bit better after I used him as a punching bag for a few minutes.