How about this. Gail gets anxious about Callum and doesn't know what to do. Whilst cleaning up around the house, she comes across a diary that belonged to her late husband Richard Hillman, which details the perfect way to kill a person and dispose of their body. Gail also finds a crowbar belonging to Richard, and uses it to smash Callum over the head when he is harassing the family.

They then bury the corpse in the manhole and cover it with concrete.