The fatal stabbing of a beloved Staten Island octogenarian is “hard on everybody,” his heartbroken son told The Post, while authorities said another man left critically injured in the Friday attack has improved.

“I can’t talk right now. … It’s tough,” the soft-spoken son, his eyes welling with tears, said Saturday morning.

Frank Pompilii, 81, was killed, and two others, including Pompilii’s pal, Redzep Cobaj, 78, and Cobaj’s son, Skender Cobaj, 51, were stabbed by a neighbor on the quiet, tree-lined Sunnyside Terrace, police and residents said.

The alleged stabber, whom neighbors identified as Ramazan Ramusevic, 57, was hospitalized after being taken into custody but has not yet been publicly identified or charged by police.

Ramusevic was “having problems,” one neighbor told The Post Saturday, adding that cops and an ambulance had “showed up two times last year and they took him away.”

The stabbings of Pompilii, Cobaj, and his son left the neighborhood reeling, with shocked residents bringing flowers and food to their grieving neighbors Saturday.

“Everybody’s confused,” said one resident, who knew both Pompilii and the suspect. “We didn’t sleep last night. … It’s heartbreaking.”

Pompilii “never harmed a fly,” the resident said. “He was always smiling, saying hello. Who would harm a man like that?”

Pompilii, Cobaj and Ramusevic frequently chatted, the resident said.

“They were always hanging out and talking. I don’t know what they were talking about but they knew each other very well,” the neighbor noted. “They always have some conversation, the two old guys and the younger guy.”

The resident would say hi to Ramusevic, who once owned a nearby pizzeria, but “he never answered back. I don’t think he was friendly with anyone else but” Pompilii.

“Frank was friendly to everyone. He was the nicest man you could ever meet – very kind very generous, always a smile on his face,” the neighbor said.

The victim was a father of two and grandfather of six who had been married to his wife, Doreen, for more than 50 years, according to the Staten Island Advance. A native of Abruzze, Italy, Pompilii arrived in America at age 17 and worked for decades making and cutting uniforms for companies on Staten Island and in the Bronx.

Pompilii was stabbed in the neck and body and rushed to Richmond University Medical Center, where he was pronounced dead.

Another neighbor, Terence Bennett, 59, said he’d known Pomilii since he himself was a teen.

The older man and his wife, “took us places, movies and stuff like that. … I’m upset. Frank was a nice guy. He was always helping people out. He took me to pick up my car last summer. I used to walk my dog and stopped and chit-chat with him,” Bennett said.

Cobaj was stabbed multiple times, and initially in critical condition but is now considered stable, authorities said Saturday. The younger Cobaj was cut on the hands and later released from the hospital.

The NYPD didn’t immediately reveal what they believe could have sparked the bloodshed.