But although he had mental health problems - an assessment in 2010 concluded he was "paranoid and delusional", and a psychiatrist diagnosed chronic schizophrenia - they were unlikely to "provide a full answer to the questions about his motivations for the siege", said lawyers assisting the NSW Coroner, Michael Barnes.
The first stage of the inquest into the deaths of Monis, Johnson and barrister Katrina Dawson, 38, who was killed by a fragment of a police bullet, has been focusing on the gunman's background and motives.
Later phases, to take place at intervals during this year, will examine questions such as why Monis was not deemed a significant security risk and why he was granted bail despite having being charged with a string of violent offences, including being an accessory to the murder of his ex-wife, Noleen Hayson Pal.
Over the past fortnight, witnesses have told of his strange and erratic behaviour, which included chaining himself to the steps of a court building where he was later convicted (in 2013) of sending abusive letters to the families of Australian service personnel killed in Afghanistan.
That was only one of a series of public protests over the years, beginning with a "hunger strike" outside Parliament House in Perth in 2000.
Monis frequently targeted Channel 7's Martin Place studio, directly opposite the Lindt cafe, showing particular rancour towards 7's Sunrise programme and host David Koch.
On one occasion, when Koch and his co-host were signing autographs in Martin Place, Monis "rushed at the talent, screaming 'you are killers and terrorists'", according to the network's security manager, Scott McIlveena.
Perhaps the most intriguing evidence came from one of the gunman's ex-girlfriends, Amanda Morsy, who had a brief relationship with him in 2003.
Morsy told how Monis - who said he was a Romanian accountant called Michael Hayson - bought her expensive dinners and showered her with gifts, including a 24-carat gold necklace. He drank alcohol and drove a Mercedes, a Jeep and a convertible Peugeot, with the plates MNH001, MNH002 and MNH003, she recalled.
He even asked her mother for her hand in marriage - but, said Morsy, she and her mother felt "uncomfortable with his presence ... There was something odd about him".
He was secretive, refused to answer questions about himself and insisted on deleting a photograph of himself taken during a family party.
Yet Monis's main concern, according to lawyers assisting the inquest, was "achieving significance", and he was "almost entirely consumed in his own self-importance".
Witnesses confirmed that. "This guy wanted to be someone," said one of Monis's many former lawyers, Franklin Arguedas. "He wanted to be a big person."
Monis, who used more than 20 different names, wrote letters complaining about the conduct of Australian authorities to public figures including the Queen and the Pope. When the Queen did not respond personally, he complained to Amnesty International.
In the months before the siege, when he had run out of money, he was being treated for mental health problems and had been charged with sexually assaulting clients of his "spiritual healing" service.
He resembled "a man spiralling downwards", said Jeremy Gormly, senior counsel assisting the inquest.
Arguedas agreed.
"His world was collapsing around him," he said.
"Maybe he went to the Lindt cafe that day to commit suicide because there was no escape for him."
