This is in case you decide to wipe out the cult with help of 4 henchmen by the bridge.
1. DO NOT flood sewers. This breaks something in the game as Ignatius group protecting the entrance to sewers is considered as part of the group in sewers. What happens? You flood sewers, save game, load that game and your party is moving to Ignatius which is not bad - but you lose ability to tell 4 henchman to leave your group which is a gamestopper bug.
2. So you kill nonchristians and got 4 henchmen by the bridge. DO NOT attack Iovita's group on the map. Avoid them and go directly to sewers entrance. Kill Ignatius and his group, kick 4 henchmen out of the party, enter sewers then kill everyone there (Gaius, Valeria).
3. Upon exiting sewers DO NOT add 4 henchmen back in your party. Now go kill Iovita's group. If you add 4 henchmen, upon killing Iovita's group you won't be able to kick them out of your party and then you won't be able to leave the map till doomsday.
Note: Rescuing the brother from the dungeon prior to all this won't break anything if you used diplomacy so Severina let you in. When you come close to her, she'll run to cells and you can, if you want, go kill her. You won't get any skill point however so… Skippit.