A Long Island man supposedly in mourning over the recent death of one pet dog, beat another of his pooches so savagely that it had to be euthanized, police said Saturday.

Michael Gallagher strangled Bella, his 11-year-old border collie mix, with a zip tie, stuffed her into a black garbage bag, and slammed a shovel on her head, police and horrified witnesses said.

When desperate neighbors — who watched the horror unfold Friday in front of Gallagher’s Levittown home — tried to intervene, he explained that he was putting the animal, “out of its misery,” a witness recalled.

“I went out with my daughter and said, ‘What the hell are you doing?’” Lisa Mabanta said. “He comes out with a zip tie, and starts putting it around the dog’s neck. I screamed at him to stop.”

“‘The dog is sick,” Gallagher allegedly replied, Mabanta said.

Just as suddenly as the violence began, it ended, with Gallagher turning around and walking away — leaving the battered animal on a heap on his lawn, Mabanta said.

Neighbors alerted Gallagher’s wife and family, and they raced home to save the beloved family pet, but could not.

“I was able to say goodbye, at least. She was a good dog, she did not deserve that. … Something snapped in my dad’s head,” daughter Kerry Gallagher said.

Her father was arrested 10:30 p.m. Friday at a nearby convenience store.

“I was at the 7-Eleven last night when he got picked up by detectives. He told me he doesn’t even remember anything. I guess he was so drunk and blacked out, he doesn’t even know.”

He was charged with aggravated cruelty to animals, and was ordered held on $150,000 cash bail at his arraignment Saturday. He did not enter a plea. A judge ordered a psychiatric evaluation.

Related Video Video length 19 seconds :19 Michael Gallagher is apprehended by police Michael Gallagher is apprehended by police Credit:

His baffled family is not rushing to the dad’s side.

“We are not bailing him out. We are not helping him,” Kerry said. “He needs mental help.”

Gallagher, a former stagehand for the Metropolitan Opera who his family said has been in and out of rehab for alcohol abuse, had never been physically abusive to people or animals and seemed fine earlier in the day, his daughter said.

Bella had bladder issues and some ear problems, but was otherwise in fine health as she approached her 12th birthday, the family said.

The family has always had dogs, including a 10-year-old pit bull named Austin, who was suffering from liver disease and was put down earlier this month.

“He loved that dog, he would take that dog every day for a walk. That dog would sleep with him on the bed,” said Karen Gallagher, his wife of 28 years said of Austin.

Michael did “cookie time” with all his pets — sharing his oatmeal cookies with them as they sat in a row. “We just don’t know what snapped, but something snapped, because it’s not him,” his wife said.

She said her husband had a drinking problem in the past, but was sober for six months — before the family had to put Austin to sleep.

The family now has one dog left: Chester, a 9-year-old mutt, who cowered under a bed during the beating.