With a single photo of his seven-year-old son holding a severed head, Khaled Sharrouf has shot to global infamy.

He has become the public face of a band of Australian terrorists who are operating in the killing fields of Iraq and Syria.

But who is Khaled Sharrouf and how did he end up as the poster boy for Western jihadism?

His history reveals a very different picture – a drug-addled petty criminal whose substance abuse led to chronic mental illness.

"He was a drop-out at school. He was ultimately expelled at a very early age for poor behaviour," former Supreme Court justice Anthony Whealy said.

"He soon took up drugs and became involved in petty criminality and it seems that part of the people he started to mix with introduced to him a very extreme form of radical Islamic religion."

It was this that led to his first serious run in with the law.

In 2005 Sharrouf was charged, along with eight other Sydney men, over the biggest terrorism plot in Australian history.

He was charged with possessing items to be used for a terrorist act – six clocks and 140 batteries he stole from a Big W store.

Sharrouf a 'very ill man', had severe schizophrenia: judge

Justice Whealy presided over the terrorism trials and heard evidence from psychiatrists who diagnosed Sharrouf with chronic schizophrenia, likely caused by his earlier sustained use of LSD, ecstasy and amphetamines.

"He was very ill mentally. He had very severe schizophrenia, he suffered from delusions - every report was very clear on this point," Justice Whealy told 7.30.

This image of a young boy pictured holding a severed head is reportedly from Sharrouf's Twitter account. ( Supplied )

"His symptoms were quite severe, he was quite delusional.

"He was overheard talking to other people when no-one was there. This was observed on quite a few occasions and it was a state of mind that persisted for some time.

"He was unfit to plead and that means in legal terms that he was simply incapable of understand what the court case would be about."

Justice Whealy thinks nothing has changed.

"Sharrouf now says, of course, that he tricked everybody, but I don't believe that for a minute," he said.

"He was a very ill man and I believe he still is."

Prominent Sydney barrister Charles Waterstreet represented one of the co-accused and observed Sharrouf in court.

"The one characteristic that I saw was that he was a class clown," he said.

"Much of his attitude and tactics with the group was to get a laugh. One put it down to dimwittedness a little bit, but he was really playing the clown.

"He would be voted the least likely to be holding a head in Iraq if one had to pick it."

Mental illness influenced radicalisation, psychiatrist says

In a report tendered to the Supreme Court, Sharrouf’s psychiatrist Stephen Allnutt, detailed how his patient's mental illness had influenced his radicalisation.

"After he realised he had a problem, he began to hang out with Muslims," Dr Allnutt said.

"They always reminded him of God. [He] began to hang out with people in the mosque, attended each time prayers were on - this relaxed him.

"He found that every time he felt paranoid, the thought of God would relax him."

Sharrouf fell in with a crowd at Australia's most infamous prayer hall, located on Haldon Street in Lakemba.

In August, 2005, Sharrouf was recorded on a listening device proclaiming his hatred for Australia.

"Forget Australia law ... Australia law get stuffed, finished ... give us all back our passports and we [sic] leave," he said.

"I swear to God I'll be the first to get out of this stuffed up country.

"Sons of dogs ... I swear I'd rather be locked up and tortured and everything in a Muslim country rather than be locked up one day in this country."

The group's spiritual leader was Melbourne sheikh Abdul Nacer Benbrika.

In 2009 Benbrika was jailed for 15 years for his part in the plot to blow up targets in Melbourne and Sydney.

Sharrouf was sentenced to a minimum three years and 11 months but, with time served, the now-convicted terrorist had only three weeks until his release.

Khaled Sharrouf battled severe mental illness as a young man. ( ABC: 7.30 )

"You have to bear in mind that his crime, although a serious crime, was a pretty pathetic crime," Justice Whealy said.

"Stealing some clocks, some batteries and potato chips from the supermarket doesn't really warrant a long time in jail."

He disagrees with people who say he should he have been locked up for longer.

"The punishment must fit the crime," he said.

"If people say that, they're probably saying [that] because they're expressing their disgust at what he's doing now, but that's not what he was charged with.

"None of us could've foreseen that he would've gone back to the radicalism that he's now displaying.

"We all hoped that he would not and there was every indication, although expressed with caution and hesitation, that he would not."

Sharrouf always wanted to join fight for Islam, friend says

On his release Sharrouf fell back into the criminal world.

Police say Sharrouf was carrying out violent extortions in the construction industry, while claiming a disability pension for his mental illness.

It was a dangerous game - a year ago his business partner, Vasko Boskovski, was shot dead in a suburban Sydney street.

Sharrouf threw himself back into hardline Islam.

He began attending the Al Risalah prayer centre in Sydney's west three years ago.

Last December Sharrouf slipped out of Australia, travelling on his brother's passport, headed for Syria.

The director of the prayer centre, Wissam Haddad, has been a close friend of Sharrouf’s for three years.

He told the 7.30 the fight for Islam was something Sharrouf had always wanted to do.

"He's fulfilling that and he was willing to give everything up to do that," Mr Haddad told 7.30.

"He never wants to come back. He wants nothing to do with Australia.

"He's happy doing what he is and he's hoping to be granted that gift from God to die as a martyr."

Justice Whealy says Sharrouf'a actions should not be glamourised or the danger he presents to Australia overstated.

"He's clearly over there playing a role of the master terrorist – but he's anything but, of course," he said.

"He's a very sad, pathetic figure. He remains a highly unintelligent man who has no perception of himself.

"The only danger he represents, I think, is that he is cast as a sort of proselytiser of this radical Islam image, and Facebook and Twitter and that sort of thing can carry that message across here to young men who might be in the position he was - young men who are angry, dissatisfied, rebellious, looking for a cause."