His job was to teach others to be safe around guns and to make guns safe for people.
But the former Victoria Police officer was forced to stand trial, accused of assaulting a colleague and friend of three decades by pointing a toy machine gun directly at him.
Mr Kerley was found not guilty on Tuesday afternoon, by a jury which took just 90 minutes to decide the case.
The entire trial lasted less than two days.
Andrew Evans told the jury he was terrified and traumatised when Mr Kerley walked into his office at Knox police station in February 2019 holding the toy gun.
"To me it was a machine gun," Mr Evans told the Victorian County Court.
"I believed I was going to be shot."
The event triggered in him a cascade of memories from 30 years in the police force. He was diagnosed with post traumatic stress disorder and took early retirement after being found unfit to continue in his role.
Mr Kerley, whose police interview was played in court, admitted he had taken the toy gun from another policeman's office and carried it into the room, where he expected to see a number of officers.
The fake weapon was used to train community members and police about safe handling of weapons.
He said he carried it in one hand, with the barrel pointed at the ground.
"I'm an expert in regard to firearm safety - my finger is never on the trigger, it's never pointed at anybody and the inference I was going to kill him ... absolute rubbish," he said.
Leading Senior Constable Dave Weller, whose office the toy gun was taken from, described the fellow divisional firearms officer as the best in the state at his job at that time.
He told the court he had spotted Mr Kerley pick up the toy and leave the room, telling him "don't point that at anybody".
Mr Kerley said he noticed Mr Evans, who was alone in the room, had seemed startled to see him holding the toy gun.
He told Mr Evans "it's alright mate, it's just a toy" and pointed it out the door, firing it to make its fake "clacking" noise.
In the interview he said Mr Evans responded "oh that's alright mate" before they moved on to a conversation about former colleagues and a reunion.
But Mr Evans, who gave evidence in the trial, gave a different version claiming Mr Kerley had said nothing to him at all and had later returned to his office and chuckled.
Prosecutor Campbell Thomson alleged Mr Kerley acted with reckless intent, knowing that pointing the gun at Mr Evans would probably cause him to fear imminent harm, but did it anyway.
In their verdict, the jury rejected that.
Mr Kerley said any belief by Mr Evans that he was going to be shot was "absolute utter tripe, rubbish".
"He's just been looking for the catalyst to get out of his job," he said.
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