On August 17, Rudy Giuliani stepped out of a black SUV outside the Fulton County Courthouse in Atlanta and pushed his way through a mass of reporters shouting the same question: what did he plan to tell a Grand Jury about his efforts to sabotage the 2020 election results on behalf of Donald Trump? Would he take the Fifth? “They ask the questions and we’ll see,” he said. When he made it to the front door it was locked — he had arrived before business hours. For 30 awkward seconds, he was cornered, laughing nervously as he waited for someone to open the door.

For anyone who has followed his long career, Giuliani’s courthouse appearance was a riveting spectacle. For much of the 1980s and 1990s, the prosecutor-turned-mayor swept into government buildings like this with an almost cinematic boldness, the most feared man in town heading into the next big battle with his entourage of dark-suited aides and plainclothes detectives. Now he is a diminished figure, angling to persuade jurors and prosecutors to keep him off the path to prison. Even if he escapes indictment in Georgia, there are two Justice Department inquiries he must survive.

It is a moment of reckoning for a man whose gleeful flirtation with danger over the decades has led him to this crucible. Perhaps none of his troubles would have emerged if he had never met Trump. Or maybe his character flaws made them inevitable.

From the start of his career, Giuliani thrived on risk-taking, the more dangerous the better. His lifelong best friend, Peter Powers, who died in 2016, liked to say that the former mayor was born without a “fear gene”. His audacity often served the public well: he was fearless in prosecuting mobsters as a US Attorney; fearless in fighting special interests as a New York mayor; fearless in leading the city after a terrorist attack.

But fearlessness has a flip side, and that is recklessness. Where fearlessness propelled Giuliani to his biggest accomplishments, recklessness loosed his destructive impulses.

His mayoral years were a dizzying tableau of heroism and heedlessness. To the city’s civil liberties and homeless advocacy groups, Giuliani’s order upon taking office in 1994 that police rid the streets of vagrants urinating or sleeping on sidewalks and in the subways was considered almost heresy. So grateful were the public, though, for the restoration of sanity in their neighbourhoods that by the end of his first term, few, if any, elected Democrats would dare vow to undo his policies.

But Giuliani's obsession with control led to a series of wildly reckless acts. He grew to resent the credit the media were giving to William Bratton, his hugely effective police commissioner, and forced him out in 1996. He then turned up the pressure on Bratton’s successors to reduce crime further, triggering a wave of police harassment of young men in Black neighbourhoods. When innocent people from those communities were killed by the police, Giuliani refused to acknowledge the problem or even meet with the city’s Black elected officials. “Maybe it isn’t an altar boy,” he famously remarked of Patrick Dorismond, killed outside a Midtown bar by officers who mistook him for a drug dealer.

Three months after Bratton’s resignation, TWA Flight 800 exploded and crashed off the coast of Long Island after taking off from JFK airport, killing all 230 people on board. Giuliani raced to the side of terrified relatives and unleashed his fury at TWA — including its chairman — for its meagre efforts to get information to them. He made it a virtual crusade against the airline, which buckled to his demands. He was widely lauded for standing up for his citizens.

The same man was cheating on his wife, carrying on at least one extramarital affair while married to Donna Hanover, who was still living in the mayoral mansion. When the city’s tabloids uncovered the relationship in 2000, the revelation blew up into a scandal that tainted his reputation for decades.

The events of September 11, 2001, all but wiped the slate clean on his pernicious behaviour in the public’s eye. As a political reporter for the television news channel NY1 covering the mayor, I ran with Giuliani and his aides from the implosion of the North Tower of the World Trade Centre. In a situation so desperate that his group didn’t have as much as a car, he rose to the occasion magnificently, proceeding to calm a terrified public with his steady leadership in the hours, days and months ahead.

Any effort to understand how this brilliant, accomplished figure could careen wildly from fearlessness to recklessness, and how the pattern led to his undoing, must begin with his view of himself as a crusader for justice, and the contortions he employs to justify his actions. Seven years after the 9/11 attacks, he ran for president and failed spectacularly, earning just one delegate for his efforts. In his concession speech in January 2008, he used a line often attributed to Teddy Roosevelt. “Aggressive fighting for the Right is the noblest sport the world affords,” he said.

At the start of reporting for my biography of Giuliani, he and I engaged in discussions over texts about the possibility of his co-operating with my efforts (he ultimately declined). In the course of those exchanges, I brought up how his view of morality had shaped his career. “My moral compass has always been clear to me,” he responded. “Sometimes I act in a politically correct way and I am lionised, and sometimes the same me comes to a different conclusion in good faith than the Democratic Regime of Thought and I am demonised.

“And sometimes you really threaten to uncover their underlying corruption and they try to destroy you.”

But he did not acknowledge how his moral compass has led him to do terrible things. In 2016, with his reputation as an American hero fading, Giuliani gambled his credibility on Trump’s presidential candidacy to revive his career. Trump’s victory produced enormous benefits for Giuliani — a return to the front pages and unlimited access to the Oval Office on behalf of his clients. The amoral new leader’s ascension gave Giuliani licence to engage in the most reckless acts of his career, with incalculable damage to himself and the country.

In 2018, Trump was under siege from Robert Mueller’s investigation into Russian interference in the 2016 election. Furious at his attorneys for playing softball with the special counsel, the President summoned Giuliani to represent him as his personal lawyer because Trump knew he relished a brawl.

Most presidential attorneys recognise the enormous sensitivity of the job and try to act with discretion. Giuliani, by contrast, found that the more outrageous his actions were, the more his boss approved.

He embarked on a bizarre effort in Ukraine to discredit the investigation, and with it the career of Joe Biden. Fuelled with a zeal to prove that Biden had engaged in a corrupt scheme with Burisma Holdings, a Ukrainian energy company, to enrich his family, he made common cause with a group of shadowy figures.

In January 2019, Giuliani played host to Ukraine’s prosecutor-general, Yuri Lutsenko, at his Manhattan office. For three days, sitting inside a glass conference room, Giuliani tried every which way to pry loose a pretext to open a Ukrainian investigation into Biden and Burisma, and to justify the firing of Marie Yovanovitch, the American ambassador in Ukraine, whom Trump saw as undermining the effort.

Lutsenko shot down each idea as impractical. It left Giuliani dejected. Six weeks after their meeting, he confided to Lev Parnas, a Ukrainian sidekick, that he was at a loss to prove his conspiracy theory. “I’ve got nothing,” he texted him.

He remained undaunted, however, even as his actions helped to get Trump into huge trouble. On the day in December 2019 that the House Intelligence Committee released its report on the scandal, laying the groundwork for Trump’s impeachment, Giuliani was in Budapest, meeting with yet more Ukrainians offering up dirt on Biden — with Trump’s blessing.

Few people in Giuliani’s position would have dared tempt fate like that. But he had no such compunctions — he took a crew from the right-wing One America News network with him and conducted on-camera interviews with his Ukrainian allies. One companion on his trip whom I interviewed for the book told me that Giuliani was so excited in his exploits one morning that he turned up the music on his iPad and sang God Bless America as he drove through the Hungarian countryside.

Unable to prove his case against Biden, he launched a harrowing campaign of character assassination. “You little slimeball, and miserable father. Hard thing is going to be not to spit in your face, the way you treat your kid, Joe,” Giuliani said on his radio show in November 2020, referring to Hunter’s addiction problems. “Instead of letting him live a life that was simple, [Joe] started to use him as a bag man to collect bribes for him,” he told RT television the same day.

By Election Day 2020, Giuliani’s fate was tied more closely than ever to Trump’s. His consulting work had dried up, his reputation as “America’s Mayor” was long since destroyed, and the people around him were growing nervous about his potential liability in the various scandals he helped spark — his companion, Maria Ryan, tried to secure him a presidential pardon. As his feelings of persecution grew, so did his fanaticism over the so-called stolen election. He became less a lawyer than a zealot.

He set out to prove the case for election fraud by any means — the final, most malignant act of his career. A man once celebrated for his rectitude and incorruptibility spewed one lie after another. At a Georgia Senate hearing, Giuliani testified that there was “more than ample evidence to conclude that this election was a sham” and took aim at two low-level election workers, accusing a mother and daughter of hiding suitcases filled with fake ballots and trading USB drives like “vials of heroin or cocaine”. The allegations were absurd (the “drives” turned out to be ginger mints). The two women were forced to flee their homes because of the death threats against them.

Behind closed doors, he was admitting just the opposite about his election claims. “We’ve got lots of theories, we just don’t have the evidence,” Arizona House Speaker Rusty Bowers, a Republican, quoted him as saying.

The recklessness had gained a stranglehold on the fearlessness. And the consequences may land him in prison.

• Andrew Kirtzman is the author of Giuliani: The Rise and Tragic Fall of America’s Mayor, which will be published September 13. It is his second biography of the former New York City mayor