Authorities admitted Tuesday that the 50-year-old self-styled sheikh "slipped through the cracks" despite the serious allegations and bizarre behaviour that should have raised alarm.
His 30-year-old wife Noleen Hayson Pal was the victim of a gruesome murder in April 2013, stabbed 18 times and set alight in western Sydney.
Monis, who arrived in Australia as a refugee from Iran in 1996, was charged as an accessory along with his then girlfriend.
Yet a magistrate said the prosecution case was weak and granted them bail, even though Monis was already facing dozens of sexual and indecent assault charges stemming from his time as a "spiritual healer".
Prime Minister Tony Abbott described the gunman as a "deeply disturbed individual", with a "long history of crime, a long history of mental instability, and infatuation with extremism".
The long charge sheet has raised questions about why he was free to mount an assault on a cafe in the heart of Sydney, in a 16-hour siege that left him and two hostages dead.
"We are asking state agencies and federal agencies to look very closely at how this offender slipped through the cracks," New South Wales state Attorney-General Brad Hazzard said.
State Premier Mike Baird added: "We are all outraged that this guy was on the street."
Monis became known to Australian authorities when he was convicted of sending offensive letters to the families of dead soldiers seven years ago. He was placed on a two-year good behaviour bond in 2013.
In addition to all of this, he had a turbulent history beyond his encounters with the judicial system, railing for years against perceived injustices against him by writing letters and challenging court judgements.
Monis had his own Wikipedia entry, detailing his various brushes with the law. He also counted astrology, numerology, meditation and black magic among his expertise, according to reports.
On his website, the self-styled Sheikh Haron also said his own children were "taken away by the Australian government" and that he was not allowed to contact them.
Showing a flair for the dramatic, a photograph said to be from his Facebook page published by the Business Insider showed him in chains with a post claiming "I have been tortured in prison for my political letters".
Monis also said the crimes he was accused of were attempts to blacken his name and likened himself to Wikileaks' Julian Assange, saying both were activists facing government-backed smear campaigns.
"These cases are in fact political cases against this Muslim activist, not real criminal cases."
The gunman converted from Shiism to Sunni Islam, according to a post on his website. Some Sunni Muslims, including followers of the Islamic State and other Sunni jihadist groups, consider Shiites heretics, or rejectionists -- "rafidi" -- of true Islam.
"I used to be a rafidi, but not anymore. Now I am a Muslim, Alhamdu Lillah," he said.
His personal website, before it was taken down Tuesday, featured a banner photo with graphic images of dead Arab children, with the caption: "This is an evidence for the terrorism of America and its allies including Australia. The result of their airstrikes."
Monis's supposed links to the Islamic State group were under scrutiny after he made hostages at the cafe hold up a black flag commonly used by jihadist groups bearing the shahada, or profession of faith in Islam.
The SITE Intel Group said Monis had tweeted frequently on news and issues related to the brutal Islamic State jihadists.
A lawyer who represented Monis in the hate mail case said he was an "extreme ideologue" but that his rampage was not the work of an organised group.
"This is a one-off random individual," Manny Conditsis told national radio. "It's not a concerted terrorism event or act. It's a damaged-goods individual who's done something outrageous."
"If you put that sort of an ideologue in a situation where he's facing extremely serious criminal charges... then from his point of view I can certainly see that he may well have considered that he had nothing to lose."