Stephen Stanko, who killed 2 and raped his girlfriend's 15-year-old daughter, is executed

South Carolina has executed a conman-turned-murderer convicted of killing two people and raping his girlfriend's 15-year-old daughter.

Stephen Stanko, 57, was executed by lethal injection on Friday, June 13, for the murder of 74-year-old Henry Lee Turner, a retired Air Force master sergeant and father of three. Stanko was sentenced to death separately for the murder of his 43-year-old girlfriend, whom USA TODAY is not naming to protect her daughter's privacy as a rape survivor.

Stanko became the 23rd inmate put to death in the U.S. this year and the third in South Carolina. He was pronounced dead at 6:34 p.m.

While prosecutors painted Stanko as a cold, calculated psychopath at trial, his attorneys at the time argued that he was insane when he committed the murders. His current lawyers argued that his life should have been spared because the execution methods in South Carolina amount to cruel and unusual punishment in violation of the U.S. Constitution, an argument refuted by state officials and rejected by courts.

Here's what you need to know about Stanko's execution.