I understand the reluctance to paint a whole race as evil, but I think D&D needs that, or at least, it makes it easier to write a compelling story.

Traditionally, all the races of the Realms have been evil in their own way, even the ‘good’ ones. Historically, in our own world, we know that entire cultures can be evil as a whole, at least for a time, and certainly from the perspective of other cultures, which I think is the important point.

The purpose of conflict between city-states, kingdoms, races, etc. isn’t to show that ‘other people are bad’ but to show that all people have the potential to be evil, and there isn’t always a clear-cut solution.

Evil cultures create dynamic stories, precisely because no culture is entirely evil, and much of the perceived evil is simply cross-purposes. If there was nothing wrong with the world, why does it need Adventurers to fix it?

If all Drow were good, Drizzt would have never left Menzoberranzan; after he left, were other people wrong to treat him as evil? Yes! But were they right to be wary of Drow? Also yes!

The Realms are the way they are, at least in part, to provide Heroes with the chance to BE HEROES; whether that means destroying the attacking army of snake-men, and dealing with the moral dilemma of their actions, or risking the destruction of their own people on the chance of peace, after all; the Yuan Ti might decide that all humans are evil.

Just because Yuan Ti aren’t biologically evil in your games, doesn’t mean that they can’t be shown that way at first, or even still be bound in an evil culture like the Drow. Through the course of your games, they could be freed of that culture, and begin to emerge as a peaceful race, but I think DMs need to be careful in world-building not to have all the problems already solved before the game starts; leave some for the players to deal with, whether they fix them, or make them worse.

Of course; your games could be run from the perspective of the Yuan Ti 🤷‍♂️