The Carta didn't just make themselves go crazy. Janeka somehow roped them into it and allowed them to become thralls to do their dirty work beyond the prison.
It's worth noting that one of the codex entries in the prison in Legacy states that the Ancient Warden's discovered that the mages in their ranks are far more susceptible to Corypheus' influence than non-mages. Apparently, Warden Mages who approached him felt compelled to free him, but after being removed from proximity, could not remember anything of what they'd said or done?
This is why Anders flips out when you take him to the prison and why Janeka was so convinced that freeing him is a good thing, because both of them were being affected by him and becoming enthralled? It's also why they required non-Wardens to seal him away in the first place, as well as recruit Malcolm Hawke to come and shore up the seals, because not carrying the taint within them would mean they'd not be affected by him?
It's probably another reason why Corypheus chose to sacrifice all the non-Mages in HLTA, as they were the ones most likely to turn against him or be controlled, once they discovered who Erimond's backer really was and what the demon army really was for?
Not to say that Janeka might not have intended to use Corypheus as a weapon initially and arrogantly believed she could have controlled him, but I'm betting by the time we meet her, she wasn't entirely in her own right mind and was probably being influenced?