If it materializes, it would surely be one of history’s most bizarre lynch mobs: hordes of worshippers of the Beatles peacenik John Lennon seeking vigilante justice.

But in recent weeks, many Lennon devotees have taken to the Internet to call for the mutilation and murder of Mark Chapman, the man who gunned down the rock star outside his Upper West Side home in 1980. Chapman, who received 20 years to life in prison for the crime, is set to go before the state parole board again October 4 – five days before what would have been Lennon’s 64th birthday. Recent press reports, suggesting the outcome is anything but certain, have prompted some fans to vent their desire to see Chapman dead.

“I think that Mark should be killed in a cruel and unusual way. I am only 13 and feel so strongly about this guy,” one blogger, identified as “Angel,” wrote.

“He did it once and should be killed! Shot! Hanged!” another Lennon fan wrote. “America is in the shape it is in because rapists, murderers and other Charles Manson losers get forgiven. If you wanna forgive these scumbag[s] …, then you forgive them; but let me and my kind kill them. I want John Lennon back.”

The campaign to murder Chapman would seem to contradict many of the core ideals Lennon preached: peace, nonviolence, forgiveness. Yet the bloodlust ran loud in Lennon’s old stamping grounds Sunday, with some people calling for Chapman’s violent death from the very foot of the monument laid out to honor Lennon and his calls for peace.

Scores of tourists and fans paused in front of the black-and-white-tiled “Imagine” memorial at Strawberry Fields to take pictures and to contemplate the singer’s legacy. Nearby, a group of musicians crooned “Give Peace A Chance” and “Imagine,” Lennon’s signature ode to a world without war.

When words turned to Chapman, however, the vibe turned sinister.

“I’d do it if I could get away with it,” said Vito Guttilla, 24, a Queens native who spoke with a fake British accent, wore his hair in the early Beatles combed-down style, and sported oversized, round shades and the white, black-fringed “New York City” T-shirt that Lennon made famous.

“I don’t think solitary confinement is enough,” Mr. Guttilla said. “This guy’s gotta pay.”

“I hope somebody shoots him in the damn head,” said a 40-something vocalist and drummer who would give his name only as “Oz.” “John Lennon could’ve given us decades of more peace, insights, and music, and [Chapman] robbed us of it. If they let him out, he’ll be dead in 10 minutes.”

The question was more complicated for those who knew Lennon personally. Walking slowly with a cane, the legendary promoter who arranged for the Beatles’ first American tour, Sid Bernstein, frowned as he contemplated Chapman’s possible release.

“It would not be justice to let him out. This guy should pay for his crime,” he said.

Declining to comment on whether he’d like to see the killer’s head on a platter, Mr. Bernstein instead answered cryptically, “I just came from Liverpool. John Lennon is forever. I just happen to believe in justice.”

Another man who knew Lennon in the 1960s, at the height of the Beatles craze, is Joey Reynolds, whose syndicated talk show is carried on the New York City radio station WOR. He said his old friend would never have sanctioned a vigilante-style killing.

“John Lennon would have practiced forgiveness. His message was peace,” Mr. Reynolds said. “It was a horrible tragedy, but nothing is unforgivable. [Chapman] served his time. Living with himself is going to be harder than living with us guys.”

Still, even those who agreed with Mr. Reynolds – and many did – said they believe that Chapman may be in for a rough time.

A guitarist who has played Beatles tunes and his own songs in the park for 25 years, John McGraw said he’s heard a lot of talk around the Lennon memorial.

“Knowing people around here like I do, knowing the love they have, the anger they have, I think somebody will be twisted enough to do something if he gets out,” Mr. McGraw said.

Chapman’s parole is anything but assured. At his initial parole hearing in 2000, Lennon’s widow pleaded by letter to keep him in Attica, where he is serving out his term.

“I am afraid it will bring back the nightmare, the chaos and confusion once again,” Yoko Ono wrote. “Myself and John’s two sons would not feel safe for the rest of our lives.”

Chapman was denied parole a second time in 2002.

But speculation has abounded that this time may be different, particularly since the high-profile release in July of Joel Steinberg from the Southport Correction Facility upstate. Steinberg, convicted of beating his 6-year-old daughter to death 16 years ago, left prison in a white limo chartered by his lawyer.

Should Chapman win his freedom, the state parole board would be responsible for arranging any security precautions.