Aaron Alexis was a practicing Buddhist but was hardly Zen — he had a hair-trigger temper that landed him behind bars several times. The Navy vet, who gunned down 12 people in Washington, DC, Monday, before being killed, was busted for incidents such as shooting another man’s truck tires and firing a round into a neighbor’s apartment. The New York native blamed the first incident on the stress of being near Ground Zero on 9/11.

Born and raised in Queens, Alexis lived on 77th Road in Kew Gardens Hills. He attended Hillcrest HS and bounced around Brooklyn, Staten Island and Harlem. In 2000, he obtained a rifle permit from the NYPD to shoot at local firing ranges, sources said. Alexis, 34, spent nearly four years in the Navy from May 2007 until 2011.

He was stationed at the Naval Air Station in Forth Worth, Texas, as a petty officer third class before he was given a general discharge after a “pattern of misconduct,” military records show. Despite the trouble, he was awarded the Global War on Terrorism Service Medal and the National Defense Service Medal. Alexis stayed in the Fort Worth area, in the city of White Settlement,and worked as a waiter in a Thai restaurant called the Happy Bowl. He spoke Thai and lived for a short time in a bungalow near a local Buddhist temple.

A temple member said Alexis could chant better than some of the Thai worshippers. But the vet was disgruntled over military dismissal. “He thought he never got a promotion because of the color of his skin,” said the fellow congregant, Ty Thairintr.

“He hated his commander.” The former reservist worked as a computer contractor for The Experts, a subcontractor of Hewlett-Packard. He moved to Washington about a month ago to begin a new HP contract and was scheduled to start at the Navy Yard this month. His position gave him security clearance that allowed him access to the facility.

The CEO of The Experts said on Monday that the company performed a background check on Alexis. “Nobody could have done anything to prevent this except Aaron Alexis,” said the exec, Thomas Hoshko. “Maybe he snapped. I don’t know.”

Alexis had spent several months this year in Thailand to immerse himself in the culture and learn the language.

A former roommate, Nutpisit Suthamtewakul, who owns the Happy Bowl, told The Wall Street Journal that Alexis was his “best friend” but a “hard-core drinker.”

“He cursed a lot, but I [didn’t] see him angry very much,” Suthamtewakul said. “He has a gun, but I don’t think he’s that stupid.

He didn’t seem aggressive to me.” He also said Alexis loved first-person shooter video games, and described him as “a 13-year-old stuck in a 34-year-old body.”

Alexis’ former Fort Worth landlord Somsak Srisan told The Washington Post that when he asked Alexis why he left the Navy, “He said, ‘Somebody doesn’t like me.’”

In 2004, Alexis was collared in Seattle for shooting out the tires of a vehicle in what police called an anger-fueled “blackout.”

After the incident, Alexis blamed it on the trauma of being near “the tragic events of Sept. 11, 2001.” His father told cops Alexis was an “active participant in rescue attempts.” He was never charged in connection with the incident.

In 2010, he was arrested in Fort Worth for firing a gun through his ceiling into a neighbor’s apartment. The neighbor said she was “terrified” of Alexis. Alexis insisted the gun mistakenly misfired during a routine cleaning. Once again, he was never charged.

He was also arrested for disorderly conduct in DeKalb, Ga., in 2008 and released on $364 bond, according to The Smoking Gun.

Only a couple of months ago, in July, a roommate called Forth Worth cops and said Alexis had put an “unknown substance” into a gas tank in an apparent attempt to damage it, the Web site said.

At the Brooklyn home of Alexis’ relatives Monday night, a brother-in-law said the killer had been estranged from the family. Still, “he was a regular guy,” the brother-in-law, Anthony Little, said. “Nobody expected this. No one saw it coming. No one knew anything. This is just shocking.”