I won't dig into the issues I personally have with the idea of escapism and fun mixed with guiltlessly killing thinking, sapient beings
Boiled down to its most essential elements, D&D is a game centered around combat (i.e., violence and death). Sooner or later, thinking beings are going to meet their end by steel or spell...well, unless you're only slaying skeletons and oozes, anyhow. Surprisingly, you can indulge in a session of tabletop roleplaying where unabashed bad guys are getting killed while your character neglects/forgets to display remorse and not be a homicidal maniac in your everyday life. If one is incapable of keeping a barrier between reality and fantasy (e.g., they find it impossible to not view monsters as humans), that is an issue they need to address; the alternative is to abandon roleplaying.
I have no issue with killing thinking beings in a tabletop game. Having villains The only time it rubs me the wrong way is the idea of just having a sapient race that can always be evil and exist to just be killed without any question of morality. Killing bandits? Sure. Killing evil wizards and their minions? Sure. (though I do find it kind of eye-rolling that such thugs would all choose to fight to the death, and when I GM a game, I usually have it that after a couple thugs have been taken down, the rest will either surrender or run away unless circumstances make sense for them not to). Also, I'm totally fine with things like devils and demons in games. Beings who are supernaturally evil because being evil is as much a part of what makes them what they are as being made of meat and bone is to making us what we are. And from what I understand about D&D lore, that goes for chromatic dragons as well. They're evil because that's supernaturally part of their nature.
You are right that making all the classic monsters misunderstood humans is dull and samey, but that's bad writing, not a product of nuance just in principle. They don't actually have to be misunderstood. Orcs and goblins can still be hostile to humans, elves, etc. And if in your setting orcs and goblins and other such monsters are all evil because of supernatural shenanigans, or because biologically they're such that they cannot coexist with elves, humans, halflings, gnomes, etc, then that's an interesting angle I'm happy to see explored. But personally, I find it hard to buy that a species that's perfectly sapient and has free will that allows them to make choices and think for themselves, would ALL be flat evil that can only be dealt with by killing them. I find that boring and unbelievable, since if they're sapient, then they can want things other than just to kill people, and if they can want things other than that, then it's possible to negotiate and come to agreements with them. It doesn't even have to be friendly. It can be a deal that comes down to "hey, this fighting is bad for both our forces. You stay away from us, we'll stay away from you." If they only exist to be hostile and kill and cannot be more than that within the setting, then they're just animals that walk on two legs.
And I also agree with you that since I'm the one that takes an issue with the idea of sapient beings being portrayed as unfailingly evil monsters, I'm the one that should be addressing it. I'm happy to see games moving away from the idea of other sapient species being just evil and I think that as a rule that's better and more interesting, but if a group wants to stick to that idea, then I just won't play with them. We just have different ideas of what we find fun, and if I tried to impose my ideas on a group that didn't share them, I would be a jerk. I have no problem finding other groups and other games that suit my playstyle better. Everything I laid out above, those are just my feelings. You feel different and that's fine. I hope you continue to find games that you can enjoy, just like I hope I continue to find games I enjoy. I don't think you're bad or wrong for your preference, I just disagree.
