NEW YORK (AP) — As his sister Victoria Gotti began a book tour befitting a celebrity author, John “Junior’’ Gotti sat in court, portrayed as a merciless killer by federal prosecutors who want to show he was far different from his entrepreneurial sister.

Last week, the government used its star witness; childhood friend John Alite, to convince a Manhattan jury that Gotti was as lethal a threat to society as anyone else in the Gambino crime family once led by his late father, John Gotti Sr.

The testimony marked the first time in four racketeering trials for Gotti over the last four years that the government had produced a witness who could so dramatically link Gotti to stabbings, murders and beatings in the 1980s and 1990s.

Prosecutors seem intent on taking a shine off the Gotti name that has resulted in part from his sister, who was on the “Growing Up Gotti’’ TV reality show and whose book, “This Family of Mine,’’ was published with a publicity blitz as her brother’s trial began last month.

Gotti’s defense attorney, Charles Carnesi, says Gotti quit the Gambino family in 1999 when he pleaded guilty to federal charges and began serving a five-year prison sentence. He also says Gotti had nothing to do with killings.

Gotti, 45, seemed unflappable through his trials until Thursday, when a US marshal told a prosecutor that he saw him mouth the words: “I’ll kill you’’ to Alite. When Alite responded, Gotti erupted, shouting:

“You’re a punk! You’re a dog! You’re a dog! You always were a dog your whole life, you punk dog.’’

Through Alite, prosecutors have portrayed Gotti as a bully with a fragile ego, someone who Alite said once stabbed a happy-go-lucky Irish friend for making fun of him during a shot-drinking game after he went to the bathroom to vomit.

“It was his ego. ... John hates losing,’’ Alite said.

He said Gotti shot another acquaintance who made fun of his small handgun.

“Is this big enough?’’ Alite quoted Gotti as saying as he grabbed a nearby rifle and shot the man in the hip.