Too bad he didn’t pursue this degree before trying to chop off his stepdad’s head with a samurai sword.

As a rebellious 19-year-old, Zachary Gibian nearly decapitated retired NYPD cop Scott Nager, murdering his stepfather in their Long Island home in 2005.

He studied behavioral science while serving a 25-year sentence at Sing Sing in Ossining — and earlier this month was among 48 inmates awarded college diplomas during a graduation ceremony there.

Gibian, now 32, told The Post that during his studies, he learned what makes people act as they do — and it was a real eye-opener.

“I think it is what most of us need,” he said. “We get to see what we didn’t see before.”

Gibian’s case made headlines because his best friend, Troy Harrelson, the son of legendary Mets shortstop Bud Harrelson, admitted helping him get rid of his bloody clothes and the antique Japanese sword.

A jury found Gibian guilty of second-degree murder, but his conviction was overturned because he was barred from testifying about his claims that Nager sexually abused him and that his mom actually killed her husband when she found out.

On the eve of his retrial, Gibian struck a plea bargain to first-degree manslaughter in which he admitted killing Nager.

He earned his associate’s degree from Mercy College through the privately funded Hudson Link for Higher Education in Prison program.

The nonprofit claims that just 2 percent of graduates return to prison within three years, compared to 43 percent of state parolees — and 68 percent nationwide.

Gibian — who finished high school while under house arrest awaiting trial — said he “always wanted to go to college” and began taking preparatory classes in 2015.

“I was in the cell blocks, and it was very loud,” he said. “You found time to study early in the day and late at night.”

His mother, father and sister watched him wear a black mortarboard and gown over his green prison clothes for his graduation inside the prison visitors center June 5.

Commencement speakers included Verdun Perry, a senior managing director of the Blackstone Group investment fund, who recalled being raised in a rough neighborhood in Philadelphia.

Perry revealed that his brother spent time in prison — and he easily could have shared that fate.

Although Gibian isn’t eligible for parole until November 2026, one of his fellow grads could be released as early as July 30, according to the prison-system Web site.

Marcus Brown, 54, was convicted of shooting another man in the gut during an argument over drug money in Harlem in 1999, then shooting a drug dealer who was fighting with Brown’s common-law wife the next day, records show.

Brown, who also earned a bachelor’s degree in behavioral science, said he was wary about returning to the streets after spending two decades behind bars. “I am scared — but it is a good fear,” he said.

His plans include seeking a master’s degree from Columbia University, which lets ex-cons attend free classes as part of its Justice-in-Education Scholars Program.