The Sith Holocaust really did leave a permanent shadow over the Sith psyche. The Sith before the Great Hyperspace War were a fairly normal civilization by Earth standards, not counting the weird space magic.
Preempting any idiots who claimed the Sith were 'clearly' evil because a) they practiced ritual sacrifices to their gods prior to the arrival of the Jen'jidai, and b) had a rigid caste society...
...a) claiming ritual sacrifice is objectively 'evil' is purely a case of cultural relativism, and which IRL was used as a justification for both cultural and actual genocide of the Mesoamerican peoples by the Spaniards and the Catholic Church. And if b) makes the Sith evil, that also makes the Indians who also have a rigid caste society 'evil', therefore we should lob nukes at Delhi and other cities before sending in the marines to 'civilize' them.
And speaking of the Jen'jidai, i.e. the surviving exiled Dark Jedi of the Hundred Year Darkness, it should be noted that while they were clearly no longer completely sane by the end of the war, at the start of the way they were just Jedi who just wanted to explore new avenues of the Force. Specifically, alchemy, only for the High Council to tell to stop. That in itself isn't bad, but what's bad is when the Jedi Alchemists left and decided to continue their studies on their own, the other Jedi followed and started killing them. In short, obey or die. Funny...I thought only Sith dealt in absolutes.
It was also the same with Xendor during the First Great Schism. They only wanted to study other avenues of the Force, and never actually started preparing for war until the Jedi Order did so first to end the schism by force. So much for keepers of the peace, eh?
Granted, they were an expansionist state led by a hereditary aristocracy that practiced slavery, which is not good, but they were hardly Nazis.
In other words, just about every pre-modern major power in RL history.
It's only later that the Sith started to go so crazy. Lunatics like Vitiate, Ruin, and Sidious never would have been allowed to take power if not for the atrocities of the Jedi.
And even in Vitiate's Empire, there were people open to coexistence with the Republic and the Jedi, such as Darth Marr. Though admittedly by then the lingering shadow of the Sith Holocaust, plus the Jedi and the Republic still clearly wanting to exterminate the Sith - again, the Infinite Army was fully sanctioned by both the High Council and the Supreme Chancellor* - means it's unlikely it could have lasted long. The Sith Empire's very existence was anathema to both.
*That's Twi'lek bitch from Taris, Saris or whatever her name was. Executing her in one of the expansions was without a doubt one of the best moments of the game.