Entering this discussion late and apologies if it's already been covered, but it seems to me that, even outside D&D, there's long been a struggle or questioning of the definition of an "evil" monster. As far back as Frankenstein (was it the monster who was evil or was it the doctor who created him?) Since then, we have King Kong, Beauty and the Beast, and most recently the movies The Shape of Water and the Eclipse series. Heck, even Gollum was portrayed as a victim of the truly evil Sauron. You have examples in plenty of modern TV Shows (X-Files, Grimm, Haven, etc) where monsters are represented as outsiders or as misunderstood and sometimes sympathetic. Even sharks, which have (since Jaws) been thought of as monstrous beasts that just love to eat humans, are starting to be observed in a less hysterical light.
The struggle hasn't been whether a civilization is monstrous but whether it is actually evil. Monsters and people can be aggressive, vicious, warlike, and predatory but perhaps not necessarily evil. Going beyond violent, predatory behaviors into premeditated evil acts seems to be independent of the "monstrous" tag, so perhaps it should always be applied separately.
Example: "I am plotting to wipe out [X civilization] because I was insulted by a schoolmate of the [X civilization] 30 years ago." This is the kind of motivation that can be provided at the individual level and perhaps explain an entire culture's attitude if it has made its way into political or religious spheres. It's the kind of thing that drives campaigns and wars, but it does seem to stem from knowing right from wrong and choosing wrong. And it's always been hard to define.
Mobs of creatures can act very differently than the individuals in that group might. And the culture and the upbringing of creatures within that collection could lead to evil individual and group behavior. But it can also produce outliers and exceptions too. It does become an interesting exception when the orcs want to parley -- you're curious why they are even doing so when it's so common for them to just attack.
I suppose the problem in D&D specifically has come with trying to do a profile of the people ("demonize") when it is realized there exists moral variation within the group. Orcs and drow specifically have had sympathetic reads on their civilizations in other media (Warcraft orcs, Salvatore's Drizzt) and there is concern that portraying any civilization with a wide brush will gloss over these exceptions and reads. That said, why not just include all civilizations in the Monster Manual and be clear that exists moral variation? You could even offer a number of hooks that discuss several ways to play the civilization with perhaps political or religious drivers causing the civilization to behave in a certain way. And then point out that there exist outliers within that overall tsunami of behavior.
Anyways, just food for thought. Carry on.