zachary davis , 17 , is on trial for the murder of his mother and attempted murder of his brother .
brutally bludgeoned melanie davis to death in her own bed in august 2012 .
then set fire to the family home in an attempt to kill his older brother josh .
the teen confessed to the arson and murder but alleged sexual abuse .
police have found no evidence of rape and josh davis , 19 , has denied it .