ALBUQUERQUE, New Mexico (Reuters) - A 15-year-old accused of killing his parents and three younger siblings over the weekend planned to continue his rampage at a local Wal-Mart store then die in a shootout with police, investigators said on Tuesday. Bernalillo County Sheriff Dan Houston told reporters that 15-year-old Nehemiah Griego, who is accused of shooting his mother and siblings before killing his father hours later, had been contemplating the killing spree for "at least a week. ...