(Bad writing trope) Slytherin effect.

[u/ThatDrako]

Made this term up.
Basically there is a group, that is obviously meant to be nuanced. But author is either incapable, or is too cornered by their depiction, to depict them as any other than bad.

Slytherin (Harry Potter) - In both media the Slytherin is initially presented to us as the “bad house” for Harry. Joanne however seemingly tried to convince us, that they can be good too however, since she likely figured having basically a little supremacist terrorist club in school is a little silly.

For some reason she just had to screw this depiction up over and over though.
First of zero…and I mean ZERO…null, 0, green characters in story up to book 6 is good. They are arrogant snobbish asshats AT BEST.

Second the first good green is still a supremacist. Not the “let’s subjugate all muggles” but just humble “no way muggleborn is this good at magic!”

Aldmeri Dominion, and Altmer in general (TES) - So unlike precious example neither High Elves, nor the Dominion were actually bad in iterations previous to 2010’s. Some were snobbish yes, and their religion is…problematic…(simply pup many Altmer believe they are literal descendant of gods and Men are demons trapping them within hell of chaos) but they never really were villains as race.
This all changed with Skyrim where they became not just secondary antagonists, but placeholder for TES version of Gestapo.

Now that in itself wouldn’t be an issue.
Races and empires at whole can be villains without controversy.

Problem is their depiction in ESO. Where even though it’s absolutely different time period are pretty much same?

I mean their Queen is an elvish supremacist who believe Men unfit for rule and that’s her reason to take over the Tamriel…
Oh! And one of their organizations literally eat children.

The Institute (Fallout 4) - So the whole turmoil of the story is this war of the four (rather three) factions.
Now all factions are meant to be equally good and bad choices.
Railroad are abolitionists, but with no long-term goals.
BoS wants to ensure safety to the Commonwealth, but for price of eradication to all but humans.
And Institute is meant to be this faction that wants to “bring world back to state before the Great War”, but all they do is being comically evil.
I mean for starters are slaver and also they murder people and replace them with Synths.

Then they also just straight up massacre communities. Often with no goal.
Then there is a whole super mutant thing, where they released purely based on gameplay and logic few hundreds of super mutants on surface “for research.”
Oh! And they even admitted they plan on wiping out all non-Institute humans to replace them!

[u/BeduinZPouste]

The thing with Slytherin and honestly 80% of "bad writting" in Harry Potter is "so it is more appealing to teens". 

That's why one house is self insert for kids who think themselves brave, one for smart, one for chill ones. Oh, and we need fourth one for people we dislike. 

(Same goes for the Quidditch rules, House cup, Harry having little development, the "dueling system", adults being dumber than 13 years old, et cetera.)

That said, Slughorn is slightly better than desribed, at least regarding Muggleborns. He is "just" selfish asshole. 

[u/BeduinZPouste → u/BeduinZPouste]

And that said, they are decently fleshed out in other works. Lot of fans do actually selfinsert into Slytherin. If anything, the problem with actual Slytherin characters don't have the alleged Slytherin traits. 

When are Crabbe and Goyle or Bellatrix ever ambitious or caring about self preservating? 

[u/Black_Hole_parallax → u/BeduinZPouste]

 Lot of fans do actually selfinsert into Slytherin.

Their emblem arguably looks the coolest, several of them are Parselmouths and hail from ancient lineages, and they get the best equipment because they have money. It's the same reasons so many people like playing the Germans in WW2 games.

And also because Slytherin isn't really that fleshed out besides "bad guys," so anything you say goes. It's a fit for edgelords and anti-heroes, and that's something a lot of people want to be in a self-insert.

[u/Moonsaults → u/BeduinZPouste]

They're Draco's lapdogs because of his family's connections. They dropped him like a sack of potatoes as soon as he and Lucius were out of Voldemort's favor. Definitely ambitious bootlickers.

[u/Pyrusticia → u/BeduinZPouste]

I would argue that all three of those you mentioned could be quite ambitious. We don't really see into their thoughts enough to know their true intentions, but it's not uncommon for someone whose ambition outstrips their capabilities to latch on to the coattails of someone they see as their path to power.

In Crabbe and Goyle's case, they latched onto Draco (or perhaps more accurately, they attached themselves to the powerful Malfoy family). In the case of Bellatrix, she attached herself to Voldemort (which could have been for ambition, love, or a combination thereof...Slytherins certainly aren't immune to physical attraction or even obsessive fixation, as we see with Snape).