I am going to say Arthas, since as a villain he literally had his soul removed and was controlled by the lich king his evil split personality The Jailer.
Depends on the canon. IIRC the "Frostmourne stole Arthas' soul" line wasn't dropped until a quest in Wrath, and WoW's writing was filled with ergregious retcons (Night Elves joining the Alliance, Forsaken joining the Horde, Kael'thas turning evil, Draenei and Eredar being retconned into being the same race, etc) . And then you have the Jailer who was never hinted at once up until Legion and wasn't even written by the same author who wrote WC3 and Wrath.
Before Arthas ever even touched Frostmourne, he had already butchered a town, never even attempting to try to save those who hadn't been zombified yet. And then there were those mercenaries in Northrend he betrayed and murdered.
Jaina and Uther bailing on him when he needed them most
Helping him carry out a massacre wasn't the right thing to do. That being said, rather than running away and allowing Arthas to do it, they probably should have arrested him.
His last sane action I'd say was the Culling of Stratholme which wasn't wrong, it was the only sane response
Some people still could have been saved. Surely not everyone ate from the newly imported shipment of tainted grain. Some people could still have been eating their old bread, or might not have been eating bread at all (fruits and vegetables, beef/pork/chicken/fish, etc).
Also, we don't know what dosage is required to turn someone. It's possible some people ate enough grain to become sick, but didn't eat enough to turn. They could have possibly been cured by alchemists or priests. But we'll never know because Arthas decided to just go ahead and kill everyone.
And I think the final straw
I agree that he is rather obsessive throughout WC3, and the final cinematic of TFT depicts him as emotionally burdened, but he was still culpable for all of the acts he committed. It's rather infuriating that Shadowlands depicts Uther as being in the wrong for having opposed Arthas. "I failed you". Excuse me? Then again, WoW has had a pretty consistent track record of victim blaming and bending over backwards to let evil doers off the hook since the War Crimes novel.
If you want Warcraft "villains" who really did nothing wrong (if you include your definition of "villain" to include characters that are framed as in the wrong by the narrative, not necessarily fought as a boss ingame), then Genn Greymane and Tyrande Whisperwind are your folks.
Genn Greymane's kingdom was destroyed. His people were murdered and enslaved by a tyrant. His son was killed.
Tyrande's people were almost entirely wiped out on a whim to spite a dying elf, her husband nearly killed by the invaders, and her contingent of priestesses were almost all killed staying behind to hold the evacuation portal open.
Sylvanas has been unambiguously evil since Vanilla, raising people from the dead and forcing them to become her slave, or killing them again. Not to mention the horrific Unit 731 style live experimentation her alchemists conducted on captives in the Undercity. She sure likes to talk about obtaining freedom from the Scourge... only for her to do everything the Scourge did, but more! Even up to trying to shoot people trying to escape from her regime to another country! And then there were her four unprovoked invasions of four countries (Gilneas, Stormheim, Teldrassil, and Kul'Tiras). She is quite possibly the most evil character in the Warcraft setting (which says a lot when you have Gul'dan, Garrosh, Sargeras, and others floating around).
People were predicting she was going to take center stage as a big bad since Cata. When Tyrande and Genn started being framed as "wrong" for hating Sylvanas (not to mention Grommash being let off the hook despite having committed a Draenei holocaust),
people began suspecting she was going to get let off the hook. Given how much Blizzard has been framing Genn as unreasonably villainous, I wouldn't be surprised if come next expansion, Genn Greymane is depicted as cackling as he exterminates the Forsaken before being killed in a boss battle. "He was unable to let go of his hatred
like Tyrande!"
Argh. Thinking about the backwards morality of the last four expansions gives me a migraine.