Moises Meraz-Espinoza walked into the Huntington Park Police Department two years ago to report a crime: He had killed his mother.Officers went to the Maywood apartment that the then-18-year-old factory worker shared with his mother, Amelia Espinoza, 42, and found a gruesome scene. A trail of blood led to the bathroom, where plastic covered the walls and floor. There, they found an electrical circular saw with pieces of bone, blood and flesh stuck to the blade. Nearby, in a freezer, police found skin and muscles stored in plastic bags. The woman’s skull, with all her teeth plucked out, her eyes...