The hare-brained plan was to outright KILL Leslie using the black flames that they wanted to sacrifice her to in ch. 1. But since those flames cannot survive long outside the shrine in which they were originally, the transported them to their state (the gazebo they were building it's a copy of the shrine and enables them to transport the flames further) before putting the tavern on black fire to kill both Leslie and Amroa.
Why Amroa went along with this? Well, it's true that her child was involved, but not held hostage in the traditional sense. You see, Amroa's husband is alive, but it's also a gambler piece of shit that SOLD AMROA'S BABY DAUGHTER to fuel his addiction. It was only needed for the Sperardos to locate the husband (in a bagrant den, no less) and he told them everything, even the baby's buyer. The Sperardos used that information, along with the promise of medicine for the baby to get Amroa on board, but as shown it definetly pains her and goes back on the plan later.
