I haven't got far into stargate so all I know is the wikis on how the ancient and ori are the same species but the Ancients were atheists who loved science while most of their race were religious and both ascended. But why are the Ori bad?
IIRC the Ori, once ascended, declared "gods did not use to exist, but now that we used secular technology to ascend to energy beings of higher dimension it means we are literally Gods now. and must be worshipped as such"
AFAIK the atheists ascended went "we must not interfere with the lesser races. let them live out as they would".
in practice it meant they backstabbed their allies who were nearly on par with them, they also left behind the stargate network and a bunch of tech. that the Gould found and used to conquer the galaxy.
basically, NEITHER the ancients NOR the Ori are good. they are both bad but different shades of bad.
The Ancients are basically USA Democrat Politicians. Militant Atheists who make everything worse.
The Ori are an alliance of:
1. Atheists who rule a nation of muslim Jihadists
2. true believers who believe
themselves to be the reincarnation of Muhammad, destined to conquer the world for their own personal benefit.
Both wanting to convert the entire universe.
Ori and Ancients are both very bad.