This does actually remind me of Suikoden 5. There's a highly secretive group of assassins that worked for the Royal Family called Nether Gate that would kidnap children in order to brainwash and turn them into assassins and an effect of the training is that it destroys the emotions of the assassin. Your bodyguard was actually rescued from them when she was very young (probably a year before she would have started killing people) and a group of recruits are actually former Nether Gate assassins that were able to have their training broken to varying degrees of success although one of them is only able to display apathy and the other has a grin that she can never get rid of.But, when I finally decided to see his content, I realized mostly stats wise he's better than Lelianna imo, and his story isn't a bad one. A good way to fix Quin seeming like an idiot for joining the cult would be to say he was born into it. Maybe Tollus has a harem and Quin is born from one of Tollus's many lovers. Maybe the cult kidnapped him and raised him in it. Much in the same way that Zevran was forced to be an Antivan Crows. Anyone of these options would work for me.
They also use a poison called raging nostrum that increases their strength and reduces their capacity to feel pain but it drives them berserk and kills them shortly afterwards.
Now I'm not saying that CoCII should be holding itself to a beloved RPG series like Suikoden but it's a lesson that the writers could learn. The cult of Tollus could do something similar with those that they are indoctrinating, have them develop Quin into someone that can charm a target and then assassinate them when they've gotten the target somewhere where they think they'd be having sex. Or they could send cannon fodder driven mad by pure corruption into causing the most havoc and damage.