After a serial cat killer in San Jose pleaded guilty to torturing and dismembering nearly two dozen felines, their former owners and friends are asking a Santa Clara County judge to impose a maximum sentence in the case that’s inflamed the quiet Cambrian Park neighborhood.

Robert Farmer, 25, pleaded guilty to 21 felony counts of animal cruelty Tuesday in a Santa Clara courtroom.

Farmer, who police suspect of killing more than 20 cats, though only four corpses were recovered, faces a maximum sentence of 16 years, said Assistant Santa Clara County District Attorney Alexandra Ellis.

The convicted South Bay resident could face anything from probation with credit for time served to the maximum at the discretion of Santa Clara Superior Court Judge Sharon Chatman.

Ellis said she hasn’t decided whether to pursue the maximum, but the “higher end of sentencing” is at the minimum appropriate for an “extremely disturbing pattern of criminal behavior.”

An attorney for Farmer, Wesley Schroeder, said he “absolutely” understands the “anger and anguish” of the victims, but he added that a long jail stint wouldn’t necessarily solve his client’s underlying issues. He plans to ask the judge for a sentence that includes therapy, he said.

“It isn’t like he shouldn’t be held responsible, but wouldn’t it be better for all concerned if a serious effort were made to try to provide the proper mental health-related services to try to ensure that this sort of thing doesn’t happen in the future?” Schroeder said.

Dozens of animal rights activists have joined bereaved pet owners to demand the full sentence.

“This past year has been very hard for all of us,” said Myriam Martinez, whose cat, Thumper, was found decomposing in a trash bin. “What really scares me is that we don’t know how long he’s been doing this.”

The pet owners are writing letters and typing up tweets, obsessively combing through Farmer’s background and mourning the loss of their furry friends on Facebook.

Through a Change.org petition with almost 30,000 signatures, they’re also laying the groundwork to lobby for a law that would establish a statewide registry for those convicted of animal abuse — the equivalent of Megan’s Law for sex offenders.

Though the FBI in January began tracking animal cruelty, reporting is voluntary for local agencies, said Lisa Jenkins, interim program manager for Santa Clara County Animal Care and Control.

“A law like Megan’s Law would be great because then when we’re processing adoptions, it would add another layer of security,” Jenkins said. “If we are going to adopt to somebody, we could quickly look them up.”

When police arrested Farmer Oct. 8, 2015, they found him in possession of a beaten, bloody cat in a garbage bag and an assortment of collars, according to court documents. Surveillance video showed a man police identified as Farmer nabbing a cat off a nearby porch.

Another neighborhood cat owner, Becky Ewens, said she and other Cambrian Park residents had noticed cats mysteriously missing. But they assumed coyotes were the culprit. And she worries it could start again if he gets out.

“I think he will continue to do this,” Ewens said of Farmer. “I don’t think he will change.”

Michael Bodley is a San Francisco Chronicle staff writer. Email: mbodley@sfchronicle.com Twitter: @michael_bodley