A teenager charged with murdering his family on newsman Sam Donaldson's ranch was a cold-blooded killer who methodically shot his father, stepmother and stepsister, then hid their bodies in a manure pile, a prosecutor told jurors Monday. In her closing argument, prosecutor Janice Schryer urged jurors to convict Cody Posey of first-degree murder despite defense claims that the 16-year-old suffered years of physical and mental abuse. (...) Defense attorney Gary Mitchell has said the boy -- 14 at the time of the killings -- told teachers, family and friends about the alleged abuse but the state failed to help him....