zachary davis was fifteen when he bludgeoned his mother 's skull 12 times as she slept and then set his family home ablaze in an effort to also kill his elder sibling josh .
on tuesday , a court in hendersonville heard how davis , now 17 , laughed as he killed his mother melanie davis in august 2012 .
davis is charged with first-degree murder , attempted first-degree murder and aggravated arson and the court was shown gruesome images of the crime scene as his trial began on monday .