The unfortunate thing about this show is that when you do look at the themes of Frieren it does support racism and ableism so the right wing nut jobs are reading it "correctly".
Both the elves and the demons are almost one to one stand-ins for Neurodivergent thought patterns.
The elves are coded as being autistic with mild inattentive ADHD the demons are coded as autistic with extreme fantasy anti social personality disorder.
The central conflict of the story is Frieren trying to communicate with others both in a way she understands and they would understand while other people only communicate with her in a way that they can understand.
They then treat her like a child when she doesn't understand them.
Her party continues to refuse to talk to Frieren by meeting her where she is until they die.
But no worries once they die Frieren learns the important lesson that communicating in the way you understand is wrong and that she needs to unilaterally learn to understand others while no one makes a real effort to understand her outside of again treating her way of viewing the world as something childish she needs to grow out of.
To be human is to be Neurotypical and anyone or anything that isn't "human" Is alien and therefore evil.
Don't worry though Frieren doesn't need to stay wrong since her race have to ability to learn to be human (Neurotypical) but this other race (demons) are incapable of learning to be human (Neurotypical) so you literally need to murder their babies before they grow up to kill millions.
A lot of people will respond to this being pointed out by saying "The fact that you see Demons and think of neurodivergent people is the problem not the writing of the show if you think real life people can be demons that's on you"
That's the thing though. I'm not the one doing that, the story those that.
The way themes work is that a story presents a character with a choice. Do a good thing or do a bad thing.
When the main character those the good thing they are rewarded when the antagonist those the bad thing they are punished.
Through this mechanism the story shows the audience that it is good to do good and bad to do bad.
Obviously outside of stories made for five year olds a story is going to be much more complex than that but the core idea still holds.
The story explains that the reason demons are as cruel and as evil as they are is because they didn't evolve empathy.
But the thing is empathy is not fantasy power source like miasma, cursed energy or spiritual pressure. Empathy is a real life thing that people can, and are, born without.
People who don't have emotional empathy in real life are said to have anti social personality disorder. (ASPD previously known as psychopathy)
When the story claims demons are evil because they lack empathy the story is by necessity claiming empathy is a prerequisite for acceptable behavior.
That is why the moment you put any thought into it the story is necessarily saying that people with ASPD are incapable of acceptable behavior and when the, story says the solution to demons is murdering them before they even have a chance to do anything wrong the story weather the author realizes it or not is implying that people with ASPD need to be murdered before they even have a chance to do anything wrong.
Going back to the elves. The elves are said to have trouble living with humans because the way they both experience and communicate time, their emotions and their thoughts are different from normal humans.
In real life people who have trouble communicating their thoughts and emotions are called autistic (and a couple other things depending on symptoms)
So when the story says that the way elves need to grow as people is by ignoring their natural way of being and adapting themselves to how normal people behave.
What the story is implicitly saying is that if you have a natural thing that makes it hard for you to communicate your thoughts and emotions to others it is unilaterally your fault and the way you need to grow as a person is by forcing yourself to communicate like a "normal person".
And before anyone jumps in with the demons aren't human we cant apply human concepts to them what I'm saying has absolutely nothing to do with fantasy races and everything to do with theme and character writhing.
You can't say my character is an orc and their fore all criticism of how the story was written is now invalid because I can't apply fantasy races with human concepts.
Where in any other media is that acceptable.
By your logic ratatouille isn't a story about balancing passion and family because the main character is a rat and we can't apply human concepts to non human characters.
By your logic Superman is not a story about trying to be the best version of yourself you possibly can and to help people because super man is an alien and their fore we cannot apply human concepts to him.
I really loved how the movie Logan portrayed the pressures of fatherhood it's really unfortunate that you now made me realize that the movie isn't about Logan raising the next generation while dealing with his own faults because Logan is a mutant and their fore incapable of having human concepts applied to him.
The Lord of the rings movies must have been about how much these non human characters love walking around with shinny objects since human concepts like bravery and responsibility cannot be applied to them.
Historical precedent
Mimics - As a class of monster have historically been used as a vehicle to express bigoted fear of the other.
The first been the concept of the changeling which in European folk lore where basically fairies that stole your children away and replaced them with Fae replicants.
Now we know that a lot of these myths related to Fae were more than likely just cultural superstitions related to autistic people.
The child who, doesn't make eye contact, explains things strangely and uses words they shouldn't know is just slightly different they are actually a evil spirit that is just pretending to be human
Now we can look back at this now with a more modern scientific understanding and laugh at the idea of fairies stealing away children but id like you to remember that being a Fae child was genuine justification to murder your children back in the day.
Another common use of the Idea of the Mimic is as a stand in for foreigners you especially see this kind of framing in the original telling of the vampire stories as well as in old school science fiction like invasions of the body snatchers.
Most modern depiction of a Mimic like creature go to lengths to divorce these creatures from their original use and vehicles for Mimicry which to me makes frieren worse.
Frieren is not Deconstructing the modern idea of mimics or demons the show is quit literally reverting it to their original bigoted stereotypes.
Dont get me wrong with what I'm about to say, I do not think that the author of Frieren is genuinely supporting what I am going to be talking about. What I am saying is that by writing the story the way he did, the connections the story will have with other things are unavoidable.
Case and point, the justification frieren uses to say the demons should be killed on sight and exterminated from the world is quite literally a reward version of the exact same argument the German psychologist who literally named Asperger's syndrome used to justify killing mentally disabled people. That they were fundamentally inhumane, that they lacked emotion or empathy, that we simply were too different to be understandable and to be integrated into society. And before a bunch of stans come at me telling me the story doesnt promote real life hate i want you to look at the very people you are fighting alongside first.
The worse part it all is how adamant fans are about defending it. The conversation is just this over and over.
"Demons are problematic because XYZ reasons"
No the author didn't intend them to be a different race in spite them literally being a different race their meant to be like sociopaths.
(Just going to let the fact the literal hundreds of Frieren fans have defended the idea that demons should be genocided without remorse by comparing them to people with anti social personality disorder sit in the air as it should)
That doesn't make it better because now rather than justifying genocide by race it's justifying genocide of Neurodivergent people. (Which to be fair is consistent with how Frieren is treated like a child for having a level of Neurodivergent that she has to "learn" To overcome as if bring different is a personal failing)
No you see demons aren't meant to represent different races or different brain types they actually represent predators.
But literally no predator would evolve the contradictory traits demons possess.
Now your just nitpicking the story isn't about evolution or humans vs predators it's a human story about people learning to have empathy for one another.
So if the story is a story about humans learning to empathize with each other what is the story trying to say by having a literal separate race that is innately evil and can't be empathized with?
No the author didn't intend them to be a different race in spite them being a literal different race their meant to be like sociopaths.
(Ad infinitum)
The demons can't be problematic because if they were my favorite show wouldn't be good so the demons aren't problematic. 
