Alright, don't quote me on it, because I'm not 100% if I really got it lol
But it seems Karas is trying to pretty much make the whole country addicted to the poison . Mataaz owns the saltmines that provide the kingdom with salt, he was responsible to execute that plan. He did so via poisoning the salt, which is distributed through all the temples.
Karas knew Ashler would try to stop him if he knew ( and probably the other 3 captains too) and that's why he gave each of them one step to accomplish the plan without them putting the pieces together to stop it. Though I'm not sure exactly how much Kin knows, seeing as Mataaz entrusted him to keep his family from being poisoned.
Mataaz himself is addicted as well, because he also had the salt at their weekly dinners between the council and Karas poisoned, and in order to not make them suspicious, he also had to consume some of it.
I read someone saying that the goal of that plan might be that they (the council) would he forced to reveal the ingredients/antidote to the black potion (which is very similar if not almost identical to the poison salt), because they can't very well reign over a kingdom of addicts in psychosis. I could totally see Karas doing that.
I hope this helped?

can someone explain chapter 107 to me like im 5 years old