Ronald Gene Simmons (1940–1990)
On December 22, 1987, Ronald Gene Simmons began a killing spree that would be the worst mass murder in Arkansas history and the worst crime involving one family in the history of the country. His rampage ended on December 28, 1987, leaving dead fourteen members of his immediate family and two former coworkers.
http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?entryID=3731
This hillbilly psycho even produced a granddaughter from an incestuous relationship with his daughter, whom he also killed. I read the book "Zero at the Bone" and was horrified at what this monster did. My brother was a true-crime aficionado too, and he couldn't get past Chapter 2 of the book without getting sick. The level of human depravity depicted in this case is unfreakinbelievable.