Something that really annoys me is that societies in D&D are either "good demi-humans" or "evil ugly people." I won't go into my hatred of D&D's "ugly = evil" aesthetic.
Why can't we portray societies of orcs, goblins, brain-eating squids, dark elves, snake-human hybrids, etc as just funny-looking humans like we do for the "pretty" dwarves, elves, hobbits, and gnomes? It would go a long way towards explaining how they can raise their own children without eating them and don't destroy themselves with paranoia and blood rage.
D&D is a game, and not remotely realistic.
Alignments were often a "team", as in the PCs were "team good" and the bad guys were "team evil" so you didn't have to hold back or do CSI or Detect Evil before you wiped out the obviously evil opponents.
The more humanlike a race, the more likely they will be not entirely evil. PCs might hold back from slaughtering orcs and goblins, especially if they have made friends with half-orcs (or there's one on their team!). A group of evil orcs isn't really different from a group of evil human bandits. You might be able to "correct" both groups.
In order to avoid situations where children are being abandoned into the wilderness (at best) adventures generally leave out orcish and goblin noncombatants. You rarely attack orc villages, instead, you attack raiding camps, where everyone is either part of the orcish military or is a civilian employee of said military. (Even orcs need clerks, even if they're all illiterate. They just have good memories.) It's not realistic to never run into these, but for the sake of a comfortable game it's probably best to avoid these.
Over time, monolithic "evil demihuman" societies show up less in adventures due to real-life societal changes, and I suspect we still see a lot of them due to inertia.
I do not believe an all-evil society is impossible. Daleks are a great example of this. (Yes, you read that right.) I got to read one of the old Doctor Who RPGs, and they went into detail on how Dalek society works. Daleks are evil to non-Daleks (or to Daleks that are different to them) but not to each other. There's no backstabbing either, unless Davros is doing something funny. The most experienced Dalek is automatically the leader, the next most-experienced ones are its lieutenants, and so forth. Of course, they have no respect for non-Daleks (or even different Daleks), so unless the PCs are Daleks (and that's not possible) they will always be opponents. The "trope" of evil always backstabbing evil need not apply.
This same kind of "nice to your own kind" pops up sometimes in D&D. Reggelids (evil spellcasters in the Dark Sun setting) have very similar rules. They automatically submit to the most powerful spellcasters among them. (They're evil because they're perfectly willing to sacrifice entire villages of halflings for more power, but it's power to their group, not to individuals.) Thri-kreen were depicted as chaotic neutral in general, but always acted lawful good toward clutchmates. Even the most chaotic evil thri-kreen raider isn't likely to backstab a clutchmate.
Dark elves being all (well, mostly) evil isn't really a problem. They are heavily influenced by their evil god, and even those that don't worship her have to hide this. Dark elves being
chaotic evil is the real problem. Evil drow would destroy other Houses but not members of their own.
Chaotic evil drow kill their own siblings, parents, even offspring, as they're all viewed as rivals. Other than the ridiculous backstabbing, an society of evil-god worshiping humans would not be much different.
Yuan-ti are biologically modified. They might literally all be lacking empathy for anyone (even other yuan-ti!), creating
at minimum extremely selfish neutral yuan-ti. And most won't be afraid of the law or moral repercussions of screwing over others to help themselves and/or their group.
Mind flayers aren't just biologically modified, they're Cthulhu. Same with beholders and many other aberrant creatures. If they're being played as "squid-headed humans" the DM is doing something wrong. They're actually worse than vampires. A vampire can at least try not to kill people (and starve, or at least spread out their bites), or perhaps only bite "evil" people.
Demons are virtually always evil and are almost impossible to redeem.
The game rules sometimes make the "pretty and good" demihumans ridiculous. For instance, if you're an evil sun elf cleric in the Forgotten Realms, who do you worship? Many sun elves are racist and so aren't likely to worship an evil human deity such as Bane. They won't worship a drow deity either. There are literally no evil elven gods in the FR pantheon, unless the DM feels like inventing one. (Warhammer does this much better. The chaotic evil god Bloody-Handed Kaine is an evil god within the elven pantheon, and is worshiped mostly but not entirely by the dark elves. Dark elves, using disguises,* can infiltrate high elf society and convert members to the worship of Khaine. Or Slaanesh.)
*They don't look much different.