Sometimes I think her original plan was to push him off a cliff when they went hiking. Not sure of the terrain of where they went, and couldn't find info about the location. I imagined that photo of the three of them was taken to show what an incredible, self-less step mother she is, before she pushes him to his death. Either she didn't think the height would kill him, or couldn't get the younger child preoccupied elsewhere to do it. In complete frustration, the next day she tries to burn him and start a fire that would kill him, but that doesn't work, and now she has an injured child and she goes into "look what you did" mode. That night she drugged him to keep him quiet while she beat him over the head with something. Then I think she stayed up, watching him like a hawk, hoping he'd stop breathing. She leaves the next day, hoping to finish him off in a remote location and leave him. Perhaps he's slumped over and moaning at this point and she's spending time at Petco and waiting for him to die. But he doesn't and she's nervous about being out in the open, so she returns home and shoots him. Still, he's alive and so she grabs something sharp and stabs him 18 times in a hurried frenzy.
This was certainly premeditated, but I really think she was trying to concoct an accident (fall or fire) where she could be deemed a woman in complete distress and a victim, and Al would come running home to embrace her and it would bring her family closer together and she'd be free of the preteen boy. When the fire didn't work and he was severely injured, she must have been PISSED. Because she knew in that moment there would be disposal and clean up and all sorts of stuff "she'd" have to endure.
