The following night, Cameron injects Petro again and sends him into the swamp. As a wolfman, he enters a nearby home and kills a little girl. Hearing about the child's death, Cameron knows his formulation works.
The following evening, Cameron injected Petro and sent him into the swamp.  As a werewolf, he then entered a home and killed a little girl. Upon learning of the child's demise, Cameron realized his concoction was effective.