Well, to be fair, the parson doubted the Apothecary when times were easy. But when things got tough, he was willing to throw away his beliefs.
As would anyone, Conor tells the monster. What else did the monster expect?
The monster says he expected the parson to give the Apothecary the tree when he asked the first time. Sure, the monster would have been killed when the tree was cut down, but the medicine would have saved both the parson's daughters and a bunch of other people, too.
Then the monster invites Conor to join in the destruction of the parsonage. To show Conor how fun it is, he busts up a settee that looks a lot like Conor's grandma's.
Totally into it, Conor helps the monster bust up the parson's whole living room.
You see where this is going, right? When the monster disappears, Conor's realizes it was actually his Grandma's living room he destroyed.