SANTA ANA – Minutes before he shot and killed his live-in girlfriend, a prosecutor said, a Costa Mesa man was drunk and listening to a Guns N’ Roses song with the lyrics: “I used to love her, but I had to kill her.”

Trial began Wednesday in Orange County Superior Court for Thomas Michael Wilhelm, 52, who is accused of shooting his girlfriend, 45-year-old Christine Marie Murray, in 2012 after their relationship and business partnership soured.

Prosecutors accuse Wilhelm of plotting to kill Murray as revenge for taking over his long-time sprinkler business, while the defense says Wilhelm suffered from mental-health issues and was clouded by alcohol and extreme emotion over the loss of his business.

He was charged with first-degree murder and faces 50 years to life in prison. Jurors must decided what his mental state was at the time of the shooting.

Wilhelm had owned his business, Wilhelm Sprinkler Co., for more than 20 years. When his business began faltering he asked Murray to step in and take part-ownership, Senior Deputy District Attorney Jim Mendelson said.

After two years, the couple began arguing intensely and Wilhelm grew distraught when Murray refused to give up the business and move out of their home in the 2900 block of Redwood Avenue in Costa Mesa.

“They both wanted the other person out of the house,” Mendelson said. “He told her on multiple occasions that he was going to arrange her to see God.”

On the night of July 8, 2012, Wilhelm broke into his neighbor’s home and stole a .32-caliber pistol, Mendelson said. Eleven minutes before the shooting, he sent a text message to a friend saying that he was listening to a song: “Used to Love Her.”

He locked the couple’s three Great Danes in a bedroom and then shot Murray six times, Mendelson said. The prosecutor said her niece, who also lived in the home, arrived back from the drugstore just in time to hear her aunt’s last words: “Tom shot me, call 911. I’m dying.”

Murray’s eight-year-old son ran next door for help.

As police arrived, Wilhelm tried to shoot himself with a shotgun but the gun misfired. Police found him splashed with blood but uninjured.

“Please tell me she’s dead,” police say he said as they led him away.

Murray was taken to Orange County Global Medical Center, where she was pronounced dead.

Wilhelm’s attorney, Doug Meyers, said Wilhelm was drunk and devastated over the thought of losing his business. Wilhelm suffers from mental-health issues and self-medicated with alcohol and prescriptions pills, he said.

Drunk and upset, he lost control and acted impulsively, Meyers said.

“Mr. Wilhelm did not have the mental state required to be guilty of murder,” Meyers said. “The evidence will show in those final moments he was not thinking clearly or rationally.”

Contact the writer: kpuente@ocregister.com