Revealing mistake: The woman leads a simple but hard life, however, in several close ups, she has long, tapered, finely manicured and polished fingernails.
Revealing mistake: When the old lady touches the door handle right before getting cut by the knife she gets paint smeared on her hand from the door handle before being cut with the knife.
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.