Timothy Parsons, who was convicted of killing his wife and mother-in-law in 2012, pleaded guilty to second-degree murder in the January prison stabbing death of Eugene Anderson.
In 2012, police said he stabbed his wife to death and then killed her mother. According to police, also he tied up a 15-year-old girl in the home and sexually assaulted her and threatened to kill her if she tried to get away and get help.
The prosecutor said after Parsons beat Anderson in Bible knowledge, Anderson apparently tried to set up a prison hit on Parsons.
Parsons stabbed him 20 times.
Parsons is already serving life but will be sentenced on his latest murder charge in September.