 -- The self-proclaimed Islamic “sheikh” and alleged sexual predator who took more than a dozen people hostage in a café in Sydney early Monday had a history of “mental instability,” Australian Prime Minister Tony Abbott said today.

The man, identified by Australian media as 50-year-old Man Haron Monis, was killed by police during a rescue operation around 2 a.m. local time after a 16-hour standoff. Two hostages were also killed, authorities said.

Addressing the nation, Abbott said that there is “understandably a lot of speculation” about exactly what happened and what prompted the incident, but he said Monis was “well known” to authorities. Monis previously made headlines for sending hate mail to the families of fallen Australian soldiers, calling the dead murderers and child killers.

“He had a long history of violent crime, infatuation with extremism and mental instability,” Abbott said. “As the siege unfolded yesterday, he sought to cloak his actions with the symbolism of the ISIL [ISIS] death cult.”

Earlier in the ordeal, hostages were made to hold up a black flag with Islamic text on it, similar to one associated with the Islamic State of Iraq and Syria (ISIS), the terror group responsible for countless acts of brutality in the Middle East. In videos purportedly made inside the cafe during the standoff, hostages relayed Monis’ demands, which included a proper ISIS flag and having politicians declare the incident an ISIS attack.

Though ISIS has called for domestic attacks in Australia as well as other Western nations, Richard Clarke, former counter-terrorism advisor to the White House and current ABC News consultant, said that he would not describe Monis as a “lone wolf terrorist” but a man with mental problems who took advantage of the publicity surrounding ISIS.

“I don't think this was a lone wolf terrorist, I don't think this was a terrorist at all. I think this was someone who was committing suicide by police as a lot of people with mental problems do, and now, if they say they're a terrorist, if they say they're somehow associated with ISIS or Al Qaeda, it becomes a major event that shuts down the city and gets international attention,” Clarke told ABC News. “This was a person with a mental problem who tried to gain attention and succeeded, tried to shut down the city and succeeded, merely by putting up a flag that was something like the flag of ISIS.”

Monis was born in Iran as Manteghi Bourjerdi and migrated to Australia in 1996, according to Australia’s 9News. In addition to pleading guilty to sending hate mail to the families of dead Australian soldiers, Australian media reported more recently that Monis had been accused of dozens of counts of sexual assault while he was working as a “spiritual healer” and was allegedly linked to the brutal murder of an ex-wife.

A website that appears to have been made by Monis or his supporters says these latest allegations are “in fact political cases against this Muslim activist, not real criminal cases.”

Manny Conditsis, an attorney who had represented Monis, told 9News he suspected Monis' mounting legal troubles may have prompted his violent act.

“All of things put together... may have put him over the edge finally,” said Conditsis, who also said that at least as of January, Monis did not appear to be unstable. “My assessment of him was that he was an ideologue. He had extreme views, [but] he never spoke of any act of aggression.”

“There was nothing [to suggest] that he would be capable of doing anything like this,” Conditsis added.

Monis’ website says he was “not a member of any organization or party” but he “supports his Muslim brothers [and] sisters… [and] he promotes peace.”

ABC News’ Mustafa Hameed contributed to this report.