Ok so, my characters wife Aela was killed by the falmer during the quest to blackreach and has vowed to kill them all on sight. But now I wish to know more about them, I know they were originally called Snow elves and lived in skyrim before the humans came. Then the nords kicked their butts and caused them to run to the dwarves for help. Then the dwarves forced them to eat the glowing mushrooms and made it so that they needed to eat it to survive which caused them to become blind and forced to be slaves. Then the war with the dwarves, them dissappearing ect. But what I want to know is why Falmer souls are considered white souls instead of black. Were they not originally human-like before? Is it even possible to change the class of someones soul? I find it cruel what the dwarves did to them and kind of regret killing so many of them.

The game lists them as creatures, not NPCs, so they have white souls like every other creature.

Lore-wise, though... I'm wondering that myself. The most believable reason would be that after centuries of being enslaved and drugged the Falmer degenrated and became mindless, bloodthirsty creatures rather than a sentient mer race. They can't even speak anymore and live in primitive tribal structures.
Keeping that in mind, I still don't believe it should turn their souls white. They used to be mer, they still have the potential to regain what they lost. Centuries of blindness and savagery shouldn't be enough to overshadow their elven ancestry...

Yea I guess it does make sense, based on what I read they seem to have only one goal, and that is to destroy the surface world. Wondering if any dlc/future games will delve deeper into this or if they will forever be mindless savages hellbent on destroying everything

There is one point during the Thieves' Guild storyline where you get a little bit of a peek into how life for the Falmer USED to be before they turned into what they are now. I'm not going to blow it for you, but it makes me actually feel kind of sorry for them. They've suffered a fate worse than the Dwemer, truth be told.

However, that's not to say I don't pound the filthy things into greasy red stains by the boatload when they run at me. I just feel kinda bad when I do it.