The ex-special forces soldier codenamed Person Two returned to the Federal Court witness box on Thursday after detailing his deployment with the former SAS corporal's squadron into Afghanistan in 2006.
He said on Wednesday that Mr Roberts-Smith expressed an alleged bloodlust while the pair were posted together in a "big bunker area" on picket duty.
"I just want to kill c****, I don't give a f***, I just want to kill c****," he allegedly said.
Person Two also recalled another time cleaning his rifle in the desert in a "safe direction," ejecting the cartridge up in the air, before catching it in his hand.
Mr Roberts-Smith walked past and allegedly yelled: "If I see you do that again I'll beat the s*** out of you".
Barrister Bruce McClintock SC, on behalf of Mr Roberts-Smith, asked on Thursday if he had ever been trained to clean his SR-25 weapon in such a way.
"No," he said.
Mr McClintock said it was a "distinctly unorthodox" thing to do.
"Probably unconventional," Person Two replied, denying it was "showboating" or "showing off".
Following their third rotation in Afghanistan and back in Australia, the troop were travelling on a bus after parachute training at Ocean Beach.
Person Two said Mr Roberts-Smith was two seats in front when he leaned across another soldier saying "hey check this out".
He then allegedly showed photographs of "dead insurgents" on his iPod.
Mr McClintock said his client didn't have an iPod with a screen in 2006.
"He did."
"You've entirely fabricated that haven't you?"
"No."
Mr Roberts-Smith is suing The Age, The Sydney Morning Herald and The Canberra Times for defamation and denies their reports that he committed war crimes and murders in Afghanistan between 2006 and 2012.
The 43-year-old Victoria Cross recipient has strenuously denied all wrongdoing.
Person Two recounted a Chora Valley mission in May 2006 where he was positioned in an observation post alongside another soldier.
Out of the corner of his eye he noticed movement, then spotted "a young Afghan boy" aged 14 or 15, unarmed and walking along a goat trail below, he said.
He believed the teenager had not seen the soldiers before Sergeant Matt Locke and Mr Roberts-Smith then "stood over us" and said "why the f*** didn't you shoot him?"
Person Two understood his specific job at the time was to provide surveillance, remain undetected, and not engage individuals if it could be helped, he said.
"I looked up and said 'because we're in an observation post'."
Mr Roberts-Smith and Sgt Locke "started running" before Person Two heard four or five shots from two distinct weapons.
In cross-examination the witness denied attending investigative journalist Chris Masters' book launch, who is also a respondent in the lawsuit.
He was asked about the journalist's notes that he described the male as "fighting age".
"I don't recall my exact words."
The newspapers which seek to rely on a defence of truth will call their next Australian witness on Monday.
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