Having played the Bastion storyline, the theory that the Forsworn are noble rebels is based on their literal propaganda. Yes, the process of becoming a Kyrian involves letting go of your memories, mostly the bad ones, but also good ones. But it's a voluntary process - one that we basically fasttrack through, it supposedly takes years on average. It is implied that if you "fail" or want to quit, you get sent back to Oribos, to presumably get assigned another afterlife. Remember, there's many more than just the 5 represented in current SL zones. There could be dozens of peaceful afterlifes where you just rest, no punishment, no service is required of you.
There's also bureaucracy/hierarchy, the main thread of the questline is about trying to get the higher-ups to notice and take seriously the threat of the Mawsworn (and the Forsworn)
The Forsworn, on the other hand, encourage people to revel in their doubts, to hold on to their memories no matter how traumatizing. And like the Sha in Pandaria, these bad memories manifest as physical (well, for the Shadowlands) and aggressive beings. One of the first transformations into a Forsworn we see is when an aspirant is possessed by, nay, lets in a memory of a Man'ari Eredar, either their former self or their torturer/killer.
The movement presumably started with the current anima drought caused by the Maw, or was even initiated by the Mawsworn on purpose.
During their assault on Purity's Pinnacle, you can see them forcibly converting Kyrians into Forsworn. You know, actual brainwashing.
They are basically Purple Smurfs.
There's also just the general structure of WoW storytelling and color coding, despite Legion's revelation that "the Light can be evil, too". The light blue guys are the "good ones", the faction that more or less a quarter of players will align with, and the dark blue guys are the "evil ones".
There's also Uther, we see him as a Forsworn, but there's also a
Kyrian version, implying he gets cleansed later. Any "twist" to explain that, like a flashback sequence detailing his origins in the "rebellion", or that the Kyrians brainwash him later to be an obediant light blue boy again, is simply unrealistic.