By Jon Herskovitz
AUSTIN, Texas (Reuters) - Texas is set on Tuesday to execute Derrick Charles, 32, who was convicted of murdering his girlfriend, her mother and her grandfather in their Houston home in 2002.
Charles is scheduled to be put to death by lethal injection at 6 p.m. CDT local time at the state's death chamber in Huntsville for strangling the three.
If the execution goes ahead, it will be the 525th in Texas since the U.S. Supreme Court reinstated the death penalty in 1976, with the state accounting for 37 percent of all executions in the country during that time.
Lawyers for Charles have filed a petition with the U.S. Supreme Court seeking to halt the execution, saying he is mentally incompetent and cannot legally be subject to capital punishment.
At the time of the crime, Charles was 19 and out on parole for burglary. Prosecutors said he sought revenge on Brenda Bennett, 44, because she was trying to stop her underage daughter Myiesha Bennett, 15 from having sex with him.
Charles hid in the family home and first beat Obie Bennett, 77, using a lamp and trophies. The grandfather was found in the kitchen with an electric cord around his neck, prosecutors and the Texas Attorney General's Office said.
He then waited for Brenda and Myiesha Bennett to come home. When they arrived, he bound and gagged them. According to prosecutors, Charles killed Myiesha and sexually assaulted and strangled her mother.
Charles pleaded guilty to the crime and was later sentenced to death.
(Reporting by Jon Herskovitz; Editing by Peter Cooney)