Let’s think about the facts.
Itsuki and the three other groups… the remnants of the church, the revolutionaries, and the missing Witch and her companions; most likely one of these groups are acting separately.
It is unclear if they are in a cooperative relationship, but there is no doubt that they are involved in this turmoil.
Each of them work on different ideologies, which means the enemy wouldn’t be under a united leadership.
In that case, Itsuki could have been given those sleeping pills and poison by another party.
While being corrupted by the curse, it wouldn’t be strange for him to ally himself with one of the groups.