At a hearing into defence force suicide in Adelaide, longtime advocate and former army corporal William Kearney said with no process for resolving that aggression, veterans often found it difficult to fit in.
"The army takes young 17-year-old kids and teaches them how to be soldiers, teaches them how to be aggressive," Mr Kearney said on Tuesday.
"They have to be aggressive to survive on the battlefield but there is no process that I'm aware of for undoing that.
"The unfortunate thing is the civilian world doesn't like aggression and that can then transform into their anger because they don't understand why they are not fitting back into this system."
Mr Kearney told the Royal Commission into Defence and Veteran Suicide that advocates were often left to deal with that anger, especially when a claim for support or assistance was rejected.
He detailed his 25 years of working with veterans, including supporting some who had experienced significant trauma on the battlefield in theatres such as East Timor and Afghanistan.
In those circumstances, advocates were hampered by having little information on what those veterans had experienced.
"There was no training where we could find out how we could relate to these people. It made it pretty difficult," Mr Kearney said.
"Some of their experience, especially the first blokes that went into Timor, some of their experiences were quite unique and we didn't know about it.
"So when they come and tell you their story, you were learning as well. So it made it that little bit harder to relate to them."
In his own army experience, Mr Kearney told how he was repeatedly advised he would never be promoted beyond corporal because of a medical condition.
He was medically discharged in 1987 after serving for 15 years.
Yet, in the context of his claim for compensation and as far as the federal government was concerned, his back issue was "healed" the day after his discharge.
In his submission, Mr Kearney made a number of recommendations including the potential for senior officers who were unlikely to be further promoted to be offered an alternative career in veterans' advocacy.
They could then be given specific training in their last year of service to make the transition that for some people "would be very attractive", he said.
In other evidence on Tuesday, the inquiry was told that a medical discharge from the defence force was clearly a risk factor for suicide among veterans.
Physician Stephen Rudzki said injuries among defence force personnel were not uncommon and were not themselves the problem.
He said the failure to respond to treatment and failure to recover was the issue.
"If you get injured and then get better, it's OK, you've got your badge of honour," he said.
"If you don't get better, people start raising questions about your veracity and your credibility.
"This leads to accusations of malingering, ostracism, and mental distress. Mental distress can lead to impulsive action."
A final report with recommendations from the royal commission is due to be delivered to the governor-general by June 2024.
The inquiry was set up in 2021 to look at systemic issues and risk factors related to military and veteran suicides.
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