They're an asocial species, yes, but they're also an asocial species that EATS HUMANS.-
(Well, actually, multiple species that eat humans, the in universe definition of 'Demon' is 'ANY monster that can mimic human speech', we see a few 'Demons' that aren't the common 'horned human' type, like Quall, or that one four armed snake guy, there are multiple species of 'Demon', and, technically, a giant man eating parrot that can mimic human words would count as one too.)
I'm not saying this as a moral value judgement, because a cat eating a mouse isn't evil, it's just eating.
Honestly, people who say the demons in Frieren are evil have no media literacy, because "Evil" actually has a meaning, and it requires "A profound Malice"
Demons in Frieren explicitly don't feel malice towards Humans, that has absolutely nothing to do with why they kill Humans at all. The idea that a Demon would feel malice towards a human is treated like a joke at some point, something only someone who knows nothing about Demons would think.
Demons kill Humans for the same reason Foxes kill chickens, Humans are food and Demons kill them basically by reflex, basically without much thinking about it.
Some make a game of it, and play with their food, like cats, but it's just something they do effectively on autopilot.
Demon's and Humans CANNOT coexist as things are now, because Demon's have such a strong impulse to kill humans and no reason to even try fighting it, because, like. Humans are food, you can't just... Not eat. And Humans will absolutely NOT accept being prey if they don't have to, and with the current magical renaissance, they don't have to anymore and they aren't.
Frieren even brings up that the Demon Kings plan to coexist with humans could have hypothetically worked- it's just that humans would go extinct long before he got close to succeeding at the rate he was going, and if she has to choose between Humans going extinct and Demons going extinct, her choice is obvious.
The actual fundamental underpinning isn't 'tolkiens orcs', it's "Deer have developed a highly effective way to kill Wolves, and now the Wolves are dying so fast they don't have time to evolve to not piss off the Deer before they go extinct."
The Demons aren't stupid, they can tell that thing's have gone wrong, that their previous status quo isn't working anymore, but they can't change fast enough and Humans won't let themselves be killed for the hundreds of thousands of years it would take for evolution to spit out Demons that have lost their "see human, kill human" instincts.
Probably doesn't help that their first few attempts to 'adjust' was to double down on what's making humans kill them, instead of trying to ween off Humans as prey, they started grouping up more to try and use numbers to counter humans numbers, but Humans are far better at the numbers and teamwork game as the Demons are, so that was a losing strategy too.
People giving Frieren shit for how she responds to Demons also seem to forget that like, 99% of why Elves are going extinct, and probably already effectively are, is because Demons ran an extermination campaign on them, which I don't think they've stopped, thinking on it.
The Demons in Frieren are fascinating, because their hunting camo is so good it's fooled a good chunk of the people reading the manga, even when it spells things out, blatantly.
It's not "the evil race of evil beings must die because they're evil", it's "a type of human hunting predator convergently evolved sapience to prey on sapients, the sapients developed the ability kill their predator in job lots, so now there are two categories of sapients, and they flat out cannot and will not stand for how things are."
Demons collectively cannot tolerate prey that won't be meekly be preyed upon, and Humans (and Elves and Dwarves) wont tolerate being preyed on now that they don't have to, so either the Demons break the humans back down into scattered, helpless prey, or the Humans kill the Demons until they stop preying on them (so, when they're all dead).
Humans in real life have done this EXACT thing almost everywhere we went, we just did it tens of thousands of years ago. The only difference is that the Demons in Frieren can talk and are smart enough to roughly understand what's happening, but not enough to see or collectively act on any way to stop it.
It's more of an environmental allegory than a racial one. We literally meet a Demon anthropologist who talked about convergent evolution and her attempts to try and understand what's happening.
We had an entire arc with Macht that showed that Demons can just barely scratch the surface level of human thought if they devote centuries of life to doing it, they're sapient beings, they CAN work it out- But the number of humans that have to die for even one Demon to do that is just... It's not worth it.
Frieren never says it's impossible, it says it's just not worth the cost.
Humans want to reach out, and sometimes other things reach back, but some things won't, in the world of Frieren, it worked out fine with Elves and Dwarves, because they're similar enough even if they're not the SAME, but it didn't with Demons.
Honestly, the biggest issue I see is that a lot of people seem to just... Refuse, out of hand, to accept even the idea that a sapient species could exist and not be exactly the same as Humans, or that you can write a story with different species without it being an allegory or subtext for race.
Frieren herself isn't Human, and trying to reconcile and understand the differences between herself and the Humans she knows and cares for is part of the core of the story. It's just that Elves don't eat Humans, so her attempts to understand Humans have a significantly less fatal consequence when she fails or makes a mistake.
Thinking on it, part of Frieren's core story is different species trying to reach out and understand each other, and their failures, struggles and successes at it. Elves feel and process emotions and time differently from Humans, and Demons are different from both of them, too, we see Frieren and Serie's attempts with understanding Humans, so to do Macht and Solitar try to understand Humans in their own ways- It's just that their ways involve killing thousands of Humans. Though Solitar probably ate more of the Humans she killed than Macht did.
Trying to claim that Frieren just says they're evil is just not reading Frieren, the story DOES go in depth into how Demons think, how they feel, how much they understand about what's happening to them and their species. The Demon King, it turns out, actually DID understand the problem and figure out the solution that would solve things if he could get it to work (Change Demon Culture and Instinctual drives so they stop killing Humans, so they can coexist without either needing to win and wipe out the other (because as things stood, Humans were going to be the winners of that game if it continued, not Demons)- He just couldn't do it.
The Demon general who can see the future apparently saw a future where Demons don't get entirely wiped out (or at least, not in the next millennia) and probably has his own post mortem plan running to that end, and we'll probably see that eventually, which will likely continue the core story beat of different species trying to understand each other.
Thinking on it, the current arc with the Shadow Warriors has made clear they're being directed by someone who can see the future... Could it be they're proxies for Schlact? Bringing an End to Magic WOULD even the playing field enough to take the pressure off of Demons for a decent while, the core cause of their current routing is Humans collectively surpassing them at magic, after all.
Given how utterly central magical power is to Demon culture, if ANYTHING could force a massive cultural realignment, it'd be losing it. Of course, I DESPISE "The Magic Goes Away" as a story concept, so...