Okay so here are some speculations i have had in my mind regarding, well what you just read in the title. Speculations on why Magic Supremacists AKA Wizarding Supremacists & Pureblood Supremacists hate muggles and muggleborns. Then come to think of it, i might as well categorize them both as Magic Supremacists because now that i think about it...Pureblood Supremacy IS essentially Wizarding Supremacy, just in a more crude and extreme form of it in a nutshell.
Okay so where shall we begin *clears throat*. Well let us assume there was a time when Wizards and Muggles did coexist, at least in some regions of the world, as Muggles did search up for individuals with magic who could easen (is that even a word?) up their problems (we have seen and heard this from real life tribes and earliest civilisations with shamans, medicinemen, exorcists etc etc) however, something did change, over time muggles became not only jealous of such powers, but also fearful of Magic (religion change in culture maybe?) and then condemned Wizarding-kind for being Witchcraft and having pacts with the devil, regardless if wizards did also convert into the said new religions (or versions of already existing religions) it was clearly NOT enough for the Muggles and the Muggles made it QUITE clearly that the Wizards were no longer welcome. And then there were the Witch-hunts, and while it is true that only like a few actual wizards perished (from what i have heard at least) the intentions of the Muggles were there and combining the fact that Wizards were "betrayed" by the Muggles, this definetly left a distasteful taste within the Wizarding Community. Wizardkind was then forced to hide, creating what...I guess is refered to as the Wizarding World, communities hidden within some achors of land which is concealed from muggles in various ways...I guess that is what they call the Wizarding World, i cannot be too sure. While the concealed achors of land did gave the Wizards protection and the home they needed, groups of wizards did overtime embitter towards the Muggles who not only "betrayed" them but also were persecuting them with the intention of killing them just because they did not understand Wizardkind. And the fact that Wizards, despite being more powerful were forced into hiding, probably did not help. And then we have the wars the Muggles were causing while the Wizards were not directly waging wars (Sure, we have like the ancestor of the Malfoys, Armand i think his name was who took advantage of le french taking control of britain during the middle ages, but that is a whole different story) no doubt created more superiority complex for the Wizards who already despised the Muggles the Witch-hunts, thinking that Muggles are nothing more than backstabbing, filthy barbarians who did nothing more than forcing Wizards to hide, causing wars etc. And by that Wizards (that ended up being Magic Supremacists) ended up believing their kind are literally superior in every way imaginable
TL;DR: Well, i think the answer is quite simple which comes in the forms of simple rhetorical questions: Why should Wizards be the ones to be forced to hide and having less land when they are potentially more superior to Muggles and could guide the world to a better place as Wizards know better? As i stated in a post where i defended Draco and i loosely re-quote it: "Aside from making it clear that what Grindelwald and/or the Death Eaters did was not ok, the series has done little to prove that Muggles are not inferior to Wizards, in fact the first Muggles we get to know are the Dursleys and we know how the Dursley treated Harry do we not?" oof that last part hits really hard when you think about it. So my only guess is that the Magic Supremacists wants revenge for the injustices that the Muggles have done to them and the world through acts of WW1 & WW2 and potentially nuclear threats during the Cold War, however, while Grindelwald and his alliance may have done things for a better future for both Muggle and Wizarding kind, Followers of Voldemort on the other hand (though it is possible that Voldemort himself did not inherently believe in such things and only pretended to believe in order to get power) just wanted revenge and dominate both the Wizarding World and Muggle one to then, well wrong Muggles the ways Muggles have "wronged" Wizarding kind via reducing Muggles as expandable slaves and what not, and thus becoming what they hated.
Now ok so we have guessed for many potential reasons why Magic Supremacists hate Muggles, from misunderstandings, to (understandable) victim-complex turned Superiority-Complex. But then why hate Muggleborns, should they not feel sympathy towards those who probably have or would go through the same things. Well, the Magic Supremacists that are more of Grindelwald type of Wizarding Supremacy might, but the Pureblood-Wizarding ones (because lets face it Pureblood supremacy is basically wizarding supremacy in a much more extreme form) however, believe that Muggleborns: as the muggleborns are the offspring of the ones forcing Wizards into hiding, well not only would be likely to reveal Wizarding World to Muggles or would turn against the Wizarding World with their powers...But also that Muggleborns are illegitamate and usurpers of the power that (according to pureblood supremacists) should only belong to them (the Pureblood Supremacists in this case)...THEM THEM THEM. Because to the Pureblood Supremacists would be the Magical Supremacists who allowed their hatred to take their beliefs of "Magic is might" to the next level that those who were not of Wizarding blood are muggles and illegimate bastards.
Still at the end of the day, it is possible that yes, perhaps Wizards are greater than Muggles but being Superior does not mean like you are allowed to go to abuse your position of screwing everyone over, it would mean that with great power comes great responsibility and the things the Magic Supremacists were doing were abusing their powers
Though to be fair,it can be argued that "Mudblood" sounds pretty pathetic for being a "slur" in-universe, i mean it can sound like something a Kindergarten-kid pirate alter-ego/OC like "IMMA cApTaIn MUdBloOD". If i was a muggleborn student i would not be offended at being called "Mudblood" i´d be more offended that they would use such pathetic lame "insult" words and be like "Is that the best you can do Maddaf----".I mean if the foul word for Muggleborn was like "Scumblood" then yeah i can get behind that of it being very insensitive and slur or if it was something like these following words: "Waste-veins of a filthy Muggle Barbarian" "Bastard-blooded muggle scum" "Dung-Blood-of-A-Muggle" "Scum of a Muggleborn" (Jesus christ, this sounds like my Death Eater OC would say as to him "Mudblood" is far too kind and i would not say he is wrong on that part, but not in a good way). *Suddenly a Death Eater Mask i Shown/flashes underneath the mask of the character in my profile picture while this plays: https://www.youtube.com/watch?v=BlSYTLYQ1yg * WHOOPS, I guess i said something that can be intepertated as me being an insensitive when it comes to fiction
