Eredar (Not speaking draenei here)
Had a utopian civilisation wherein they could have shone light on other civilisations or simply continued as they were peacefully.
Takes offer of power and smashing other civilisations to the ground under pretence of a necessary crusade. I mean they may believe it’s “right” but the way they go about it? Ripping people’s souls out? Burning them alive? Various claims about feasting on their essence/spirit and revelling in their misery?
They are an example of a race that had everything in them to be good and instead they decided en masse to use those gifts for the purpose of destroying everyone else and that makes them completely evil. Worse still- they believe what they are doing is justified and that the various planets they’ve reduced to ashes is “justice”.
Even Forsaken have individuals capable of showing sound judgement or understanding when a line is crossed. More to the point the Forsaken had their unique situation thrust upon them, outside of their choice, and there were consequences on their outlook.
The Eredar willingly walked into the “evil factory” simply because the guy asking them to told them destroying planets was necessary and good. You have to be pretty ethically bankrupt to not have any issues with doing that and instead growing to love it.