Don't believe the common misconception that Slytherin's main deal is being "the evil house." It couldn't be further from the truth - there is nothing inherently evil about Slytherin. In fact, their house traits are all positive ones, albeit largely selfish or self-serving:
Nowhere in that list will you find "evil" or "wants to murder and/or subjugate the entire Muggle population of Earth" or "wants to enact a massive campaign of ethnic cleansing, even though he can't even take over a GODDAMN HIGH SCHOOL." Do the qualities that often lead to individuals turning to a life of evil sometimes coincide with some of these traits? Sure - you'd be hard pressed to find a dictator that couldn't be described as "ambitious", but do you know who else was ambitious? Martin Luther King Jr. Mahatma Gandhi. FDR. Susan B. Anthony. It's silly to argue they would have been sorted into Slytherin - the houses are designed largely so that you could make an argument for pretty much anyone to be sorted into any particular house - but I would certainly say they were all resourceful, cunning, ambitious, determined, etc.
And the proof is in the pudding - while many are mostly familiar with the evil wizards Slytherin produced, there were also just as many (if not more) good, honorable, worthy wizards who bore the silver and green of the house - including:
The problem is that the bad guy wizards get all the press, while the good guy wizards who DIDN'T turn into Wizard Nazis don't get headlines written up about them (and given the relatively small number of Death Eaters we see, it certainly seems like a minority).
Okay - let's not beat around the bush TOO much: Slytherin has produced more dark/evil wizards than any other house. That is a fact. Additionally, they've produced THE MOST EVIL wizard of all-time: Voldemort, an incredibly evil, powerful wizard who nearly brought the world to its knees, and performed levels of dark magic previously thought impossible.
And it's impressive as hell.
Regardless of good or evil or any measure of morality, Slytherin produced wizards with enough raw power, smarts, and cunning to come damn close to conquering the wizarding world. They were aiming for horrendous ends, but there's no denying that Slytherin grew them into incredibly potent wizards. No individual Hufflepuff, Gryffindor, or even Ravenclaw has come close to accomplishing anything near what certain Slytherins have done. Something about Slytherin House's focus on ambition and resourcefulness pushed their students beyond what anyone thought possible. On some level, you have to respect that.
Prior to incredibly-lucky, constantly-assisted-by-others, barely capable scar-haver Harry Potter and his crew showing up, Slytherin was the reigning champ of the House Cup competition for seven years running. SEVEN YEARS STRAIGHT - that's a student's ENTIRE tenure at Hogwarts' worth. How were they able to win so consistently? Because they're the best - and without the unfair variable of professor's pet Harry Potter randomly generating bonus points for Gryffindor based on Dumbledore's whims, Slytherin was the deserved winner.
The biggest negative to Slytherin (beyond all the Wizard Hitlers they keep mysteriously producing) is likely the "racist" aspect - namely, that they only accept "pureblood" wizards (or at least, vastly prefer pureblood) and harass mixed-blood wizards. This is, at least, the vibe you get from the books and films - since Slytherin is largely represented by Draco Malfoy and his crew, all of whom are unrepentant racists.
It all goes back to the House's founder, Salazar Slytherin, who was also extremely racist - he not only made it clear that his preference was pureblood students, but he was the architect behind the Chamber of Secrets, a hidden part of the school that housed a basilisk, whose chief goal was to purge Hogwarts of any Muggle-born wizards.
So yeah....that guy was PRETTTTTTTY racist.
And while his legacy certainly still exists, not all of the students in Slytherin share his views. And notably, many of the most famous graduates of Slytherin are half-bloods, including Severus Snape, Dolores Umbridge, and Tom "Voldemort" Riddle.
It's kind of like a relatively recent Simpsons joke about Fox News - the organization itself isn't necessarily racist, it just happens to be very popular with racists:
Okay this one isn't exactly bolstering my case, maybe ignore it.
What has lions as logos? Like, everything. It's boring. What has eagles as logos? There are like twelve football teams that have some similar-ish bird of prey as their logo, so c'mon. That's some real normie shit. And badgers? Despite what I may have previously said about their badassness in combat, they look like tiny little dorks.
Basically, each logo for the Hogwarts houses suck....except Slytherin's. Beyond the awesome green and silver color scheme, having a snake as your logo is something truly daring. Snakes are rarely used as logos, because of the inherent association with underhandedness and deceit that snakes have. Slytherin knows that too, but they EMBRACE it. They take that unsavory association and they decide to fully own it, loud and proud. They're not trying to appear noble or any other kind of pretentiousness that marks most of the other logos - they're being brutally honest about who they are, and how they refuse to apologize for it.
And that's Slytherin in a nutshell - they know exactly who they are, and they're not going to change themselves to please others. They're in it for themselves - and to reshape the world around them to fit their vision, and not the other way around.