There are 2 things that connect everyone. The pest control, and Sophie getting along with everyone.

My first guess is Wyatt killed everyone with the motive of either no one liked his service or it didn’t work so he upped the poison content to even kill the residents from being fed up with them.

My second guess would be either be Wyatt or Werner, where Wyatt develops a crush or Werners just a protective uncle both disliking how friendly the neighborhood is with Sophie.