Lets talk about Vex...
!!SPOILERS FOR SEASON ELEVEN OF NINJAGO!!
!!TW: Mentions of Death, Mass Murder, and Enslavement. If any of those trigger you please scroll past.
(Note: I am going off of my recollection of Season Eleven, and the Ninjago wiki. If my facts aren't all straight, please lmk respectfully ^^)
So. In my opinion, Vex is by far the darkest ninjago villain that we have seen so far (I have not seen season 15 yet, and I skipped season 12, so I don't have anything to say about either of those villains) followed by The Skull Sorcerer, Nadakhan, and then the Chen/Clouse duo.
When I say darkest, I don't mean evilest. Pure evil would be evil just for the sake of being evil, and we have the Overlord for that. No, when I mean darkest, I'm talking about which villain is the most....well, dark. Which villain just makes your skin crawl the most due to how fucked up they are. That's what I mean by dark.
So, lets start off by talking about his various war crimes.
So, lets start off with his treatment of Zane, one of the most beloved characters in the entire community. Upon learning of Zane's ice power, he was willing to go out of his way to obtain them in whatever way possible. He then made Zane lose all of his memories, and manipulated him into becoming the Ice Emperor, who then went on to commit atrocities that Zane would never even think of doing, even in his darkest of thoughts. He took Zane, the kindhearted, heroic, and lovable Zane, and turned him into an evil dictator emperor.
Now, lets talk about what happened after that.
I do not blame Zane for anything that happened while he was the Ice Emperor because he was under the influence of Vex, and the Scroll which had corrupted him. While Vex had Zane under his control, he manipulated him into committing FUCKING GENOCIDE, both attempted, and actually committed. Vex succeeded in killing off the entire Yeti species besides Kraig, who was the only yeti alive after the murder. He attempted to murder the entire People of Great Lake by putting out their fire, and having them starve/freeze to death. If it wasn't for Kai, they would have probably slowly frozen to death.
Aside from that, he also overthrew the current monarchy, and basically enslaved them (Blizzard Samurai, Blizzard Warriors), he held both Akita's brother and Lloyd hostage in the freezing cold dungeon, (he was originally going to kill Lloyd) and he also tried to manipulate the monarch before him, Grimfax, to attack the innocent Formlings.
All because of his paranoid delusions.
Now lets move onto his motivations. Vex, as you know, is formless. He abandoned his people, the Formlings, some time prior to not finding his form. He was driven mad by his insecurity, falsely believing that his people had begun to turn their backs on them, and he abandoned them some time after. He swore "revenge" on his people, despite them not actually hating Vex, and stopped at nothing to get that revenge. Before he tried manipulating Zane, he tried his hand at Grimfax. Luckily, he refused.
Lets just unpack this...
There is no doubt in my mind that the events of season eleven severely traumatized Zane. All of what I mentioned above (except for that last paragraph) was technically committed by Zane, who was being manipulated by Vex and the Scroll. Absolutely nobody would come out of that unscathed, especially someone of Zane's character. Zane's character has been established from the very beginning as the soft spoken, kindhearted ninja who's main goal in life is to protect others who cannot protect themselves. This character had his memories forcibly erased, and was forcibly corrupted by an evil scroll and "royal advisor" who then forced him into committing all of what I listed above. It's just not realistic or even possible for Zane to come out of that completely 100% fine. It's honestly a shame that Ninjago really didn't touch on that much after season 11 ended. It would have brought forth a great character development opportunity, and it would have provided a much more realistic aftermath to the season.
There's also no doubt in my mind that this would have been traumatizing for the other ninja as well. I mean, imagine having to see one of your loved ones being manipulated into being a horrific monster, who was responsible for some of the most horrific events you had ever heard. It would certainly fuck with your psyche at least a little bit.
However, back to Vex...
Vex (very possibly) gave so many people severe trauma. I mean, he enslaved the people working under Grimfax, the ruler who came before the Ice Emperor, he corrupted Zane against his will, he killed off Kraig's family in one night, leaving him completely alone, he terrorized the entire area for DECADES, and he launched a huge attack on the people of the Lake Village.
In conclusion, Vex no shit severely damaged and traumatized an innocent fucking nindroid with a heart of pure fucking gold, by erasing his memories, and manipulating him into committing fucking atrocities such as GENOCIDE AND SLAVERY all because of his thirst for revenge, that was sparked by a paranoid delusion based off of pure insecurity. Vex is by far the darkest, and in my opinion, one of the most evil ninjago characters/villains in the entire show. I am not at all surprised as to why the fandom hates him, I do as well.
Anyways, that was my analysis on Vex and his character. If you would like to add anything or criticize me, please talk with me in the comments or DMs! But please be nice, and please use tone tags! I am very sensitive, and I can't read tone over text that well!!