PORTLAND — A man who is serving 60 years in prison for strangling his girlfriend, cutting off her head and setting her body on fire was not insane at the time and is criminally responsible for his conduct, Maine’s highest court ruled Tuesday.

In a unanimous decision, the Maine Supreme Judicial Court rejected Chad Gurney’s appeal of his murder and arson convictions in the killing of 18-year-old Zoe Sarnacki of Portland on May 25, 2009.

Superior Court Justice Roland Cole rejected Gurney’s insanity defense in last year’s trial and sentenced him to 50 years for murder and another 10 years for arson.

On appeal, Gurney’s attorney told the supreme court justices that Gurney, 30, wasn’t criminally responsible because he was suffering from mental problems and having a psychotic episode that made him delusional.

Prosecutors maintained that the evidence showed Gurney knew what he was doing when he killed Sarnacki in his apartment in Portland and knew it was wrong.

No one disputed that Gurney was never completely normal after being hurt in a bad motor vehicle crash in 2005 or that he has a “peculiar” personality, but those things didn’t make him insane, said Deputy Attorney General William Stokes.

Even if Gurney were mentally ill, that wouldn’t mean he was insane at the time of the killing, Stokes said.

“The mere fact that you have a mental disease or defect — and I’m not so sure he did — doesn’t answer the question of insanity, because you have to lack the substantial capacity to appreciate the wrongfulness of your conduct,” Stokes said. “People with mental illness can appreciate right from wrong. They do all the time.”

Gurney’s attorney, Sarah Churchill, did not return a phone call for comment.

The facts of the case were never in dispute, but Gurney’s mental state at the time was.

During the trial, the defense contended that Gurney’s mental health problems started at Liberty University in Virginia when his lacrosse team’s 15-passenger van crashed in 2005. He suffered a severe head injury and other injuries that required 20 surgeries. He later returned to Maine and lived off a multimillion-dollar insurance settlement.

Gurney’s attorney said Gurney’s psychotic break stemmed from a combination of his head injury and withdrawal from prescription medication. A psychiatrist who first evaluated Gurney, more than a year after the crime, testified that Gurney suffered from a psychotic disorder and couldn’t appreciate the wrongfulness of his actions.

But prosecutors said Gurney was simply angry because Sarnacki had slept with someone else while he was out of town and later refused his request to quit her job at a bagel shop and join him on a trip to Thailand, where he hoped to further his interest in Eastern religion.

In its 20-page ruling, the Maine Supreme Judicial Court said Justice Cole found the reports by physicians and psychologists who examined Gurney closer to the time of Sarnacki’s death more credible than that of Gurney’s expert.

The justices also rejected the defense’s claims that Cole erred in denying motions to suppress evidence found on Gurney’s Facebook account, his laptop computer and cellphone, as well as a reference to a video about beheading that was found on his computer hard drive.