If you could kill any character by any method.
Personally I would kill Magni by hitting him in the face with the Heart of Azeroth so hard that his head shatters into a thousand pieces.
EDIT: You can do the deed yourself or have another lore character do it for you, of course.
Tyrande. I would slowly submerge her in a vat of sulfuric acid
Myself, in game.
Nazgrim can do the deed.
Nathanos… but I wouldn’t kill him. That would be rude and well, he is already dead. Instead I would lock him in a room for an eternity of torment with the Teletubbies.
Also magni, throw him in such magical realm of eternal suffering which also erases his existence from all of our memories.
Tyrande and the only way is obviously with fire 
I would let Nathanos board the new Horde Zeppelin “Hindenburg” en route to Ogrimmar.
Nathanos. Lock him in a box, put that box in a bigger locked box, and then throw the box into a volcano. Roast some marshmallows.
I would kill Sylvanas multiple times.
Anduin gives his life to trade places with Garrosh, returning him to his body.
So, Garrosh is now a human and bent on exterminating the enemies of his race?
Yeah, okay, let’s do that.
I’d like to kill sylvanas
A daily quest where therazane the stone mother will sit down on sylvanas’ head and squash it like a pumpkin.
The daily quest is required to be completed 10 times in order to unlock the second part where you take the corpse and throw it away on a pile of manure called, “the loyal pile”. This way we can integrate the expac poop quest with the end of an annoying character.
That sounds perfect! She is no better than c**p anyway sorry I meant worse.
Anduin.
Strangled with my bare hands while he argues his guards down in favor of attempting to establish peace between himself and I.
Scrollsage Nola. She’s the REAL villain of this expansion with her crap “entry level to programming videogames” level minigames.
Malfurion. Big useless jerk is useless. He should just go back to sleep and have green dreams.
Omg ikr! She talks I’m like SHUT UP! (A turtle made it to the water!) The stupid calligraphy world quest. (Hmm I’ve seen worse but I’m sure you can do better.) WHAT DO YOU MEAN THAT WASN’T GOOD ENOUGH!
Anduin.
My chimaeras and night sabers gotta eat yah know?
Behind the veil of a humble purveyor of baked goods, there is a monster, a reprehensible sadist the likes of which this world has never known!
the devs of bfa and whoever came up with pathfinder for flying.