People didn't fall in love with the Joker or Broly because they were super "deep" and "complex". We like them because they're fun to watch and it's super satisfying seeing their ass getting kicked. Hopefully, Mordekaiser will be full-mode Runeterrian Sauron and not have a tragic a backstory like someone destroyed his sandcastle and now he will make a bigger castle with the bones of his enemies as revenge.
[vlad-salute]
Currently all pure evil characters come from Ionia
They all find ways to break the game
I think you're mistaking the point that Riot's writers have been trying to push over the last couple months. Having a deep character doesn't mean they need to be a tragic character, but evil for evil's sake has its strengths and weaknesses. It's not about "Mordekaiser was abused as a kid and wants revenge on the world." but it's about the deeper motivations behind them.
For example, Noxus is our example of a Meritocracy, a place where you're judged on your ability, and achievements. Power too, since that usually shows itself in being 'The best'. Mordekaisers simple philosophy could be extremely simple, but pose a deeper question. He's a believer in the Meritocracy, but to such a far and radical extent, that he believes the world should be ruled under his boot, because, he is the most fit, accomplished, and powerful to rule it. And if he loses along the way, it wasn't meant to be, but that won't stop him from selling his soul in order to pursue it.
Ideologically, he shares the same philosophy as the common Noxian, but it's transitioned so far into radical extremes, that it's almost unrecognizable. Without making Mordekaiser tragic, you're left to question the downsides of the Meritocracy philosophy. Is it right for the weak to be trodden upon, because they're weak? Does it excuse cruelty and malicious acts if it proves that you are the most accomplished, the most powerful, the most capable to rule? You've made him a deeper character, without the need for sympathy, or making him truly 'evil for evil's sake'.
The final option in depth: What is a kingdom under Mordekaiser like? Would those he conquer, if they choose to serve the most powerful, and capable, leader, live earnest and good lives? Is it okay to conquer and murder the world as long as you're a fine ruler of it afterwards, as long as you're the one in control? Are they oppressed 24/7 to prevent rebellion? Why would he keep those oppressed, if not because of a character flaw within him. Paranoia that someone will come to surpass him, a belief that those who can't rise should make the ground upon they walk, something that a better writer could think up, we dunno.
The mistake I keep seeing here, is that Pure evil = Simple, which isn't the case. Even Sauron, in his rise of top-villain status, was the result of a fallen Maiar. It's completely fine to have a character who wishes to destroy the world, but it has to stem from a solid foundation outside of pure evil, like a misshapened belief that 'this is the only way for humanity to survive' or something deeper. You don't need tragedy to make a solid villain, but it helps make them relatable. And that's what we want, relatable villains.
I mean, 
is basically pure evil now.
He is hellbent on taking over Shurima and enslaving the population for all eternity as its true emperor, and he does magic mind rape to that end, wrecking cities and people that dont fall over on their faces when he goes near.
There is 
who is basically the league edition of the joker, a nutcase who practices "art" that involves mutilating corpses and decorating the surroundings of said corpses while delighting in the reactions of the people who stumble on his "masterpieces" afterwards.
are all also pretty damn nuts in the current lore, Volibear eats souls for snacks.
Then we have what i´d describe as the Machiavellian evil 
, the man has 0 qualms about using necromancy to raise the dead heroes of the nation and have them keep fighting, anything to win.
The insane&possessed evil 
Very ambiguous cult leading evil.
And the lord of evils 
.
Hopefully, Mordekaiser will be full-mode Runeterrian Sauron and not have a tragic a backstory
If he even does, I cant imagine that being so dreadful of a backstory anyway. So what would it matter. A quote-un-quote tragedy wouldnt lessen the quality of the character like your trying to suggest.
"People didn't fall in love with the Joker... because they were super "deep" and "complex"."
Don't let the Joker fangirls hear you, they will rip you apart if you say he's anything but a misunderstood genius, it interferes with their Batman fanfics.
Peoples who hate Wukong x Ahri are evil.
I liked the begginning, but really disagree with Thanos being poorly written. Sure, sometimes he could make contradicting character actions, but people in real life do that too all the time. The video started getting repetitive after a while and I think some examples could have been cut out and keep it at the 14 min mark.
Also Hades from Disney's Hercules is a great simple villain you didn't mention. I think ultimately for video game characters if the character is fun you will play him/her. Some of the most popular characters Lol has to offer (like Teemo) really have no motivations at all behind him, but people find him fun for whatever reason. I think many times writer get over their heads trying to create something they think is good work because they want to push themselves, and end up overcomplicated
is definitely coming up pretty high, for a human.
People didn't fall in love with the Joker or Broly because they were super "deep" and "complex"
I feel like Broly is a pretty poor example here as original broly is extremely controversial in the db-fanbase and a lot of people actually dislike him because his character is so dumb and simple. In fact the new version of him from the Super Broly movie seems to be overall more liked because he is a deeper and more complex character.
If you wanna go for a dragon ball related example, I mean the guy is one of the most iconic villains of all time and probably one of the most evil beings in all of fiction and who doesn't have anything sympathetic or good about him.
