i'm not an expert on the lore, but the forgotten realms wiki and the recents source books explain a lot, there is even novels about Yuan-ti being nice and not stupid assholes defined by their nature. just think about the real world with people changing there ideas of other people over time (like slavery of black people existed 2 hundred ago, now it doesn't) like in the real worlds some event happened to move the lore, nothing must be set in stone, or dnd would have been given upon a long time ago, you would have the same bland situation over and over a game, that is what i call pretty boring.
There are so many things wrong with this worldview that I can't even begin to describe it...