A Costa Mesa man was convicted Wednesday of stealing a gun and shooting and killing his live-in girlfriend, who also was his business partner.

A jury found Thomas Michael Wilhelm, 52, guilty of one count of first-degree murder with a sentencing enhancement for personal discharge of a firearm causing death, according to the Orange County district attorney’s office.

Wilhelm faces a maximum sentence of 50 years to life in state prison. His sentencing is scheduled for Feb. 3 at Orange County Superior Court in Santa Ana.

On July 8, 2012, Wilhelm went to a neighbor’s home on Redwood Avenue and stole a handgun, according to the district attorney’s office.

He returned to his home and shot his girlfriend, Christine Murray, six times.

After running out of ammunition, Wilhelm returned to the neighbor’s home, stole a shotgun and tried to shoot himself, but the gun fired into the ground, prosecutors said.

According to the district attorney’s office, Murray, 45, sent text messages to a friend 11 minutes before she was murdered that said Wilhelm was intoxicated and singing along to the Guns N’ Roses song “Used to Love Her,” which features the lyrics “I used to love her, but I had to kill her.”

When police arrived minutes after the shooting, they found Murray unconscious in the bedroom with several gunshot wounds to her chest.

Murray, the mother of a then-8-year-old son, was taken to a hospital and pronounced dead.

The defense argued that Wilhelm had mental-health issues and was extremely emotional as the couple’s personal and business relationship soured.

Authorities said Wilhelm and Murray’s three-year relationship had faltered during the two years before her killing after he brought her in as a partner in his business, Wilhelm Sprinkler Co.

According to a transcript of Wilhelm’s grand-jury indictment, he had a history of drug and alcohol abuse and Murray had begun to question his abilities as a business owner.

benjamin.brazil@latimes.com

Twitter: @benbrazilpilot