The WA Premier says language used by Clive Palmer created an "anger and madness in the community" that contributed to people targeting his family with death threats.
Mark McGowan has started giving evidence in New South Wales in the defamation case brought against him by Mr Palmer.
He is countersuing with his own claims against the mining magnate in the Federal Court case.
Mr McGowan told the court comments by Mr Palmer that he had acted corruptly "cut to the core of what he was about".
The comments were in the context of legislation preventing the mining magnate from claiming billions of dollars in compensation over a mining proposal, passed by the WA Parliament in 2020.
"It was a deep wrong against my character, particularly in the context [that] I was trying to protect the state," Mr McGowan said.
'Crazed language and behaviour': McGowan
Mr McGowan said the suggestion he had behaved corruptly was "deeply offensive", and gave rise to actions that meant he had a police car parked outside his house 24/7 and was forced to close his electorate office over safety concerns for his staff.
He said these included threatening to kill his children, ramming a car into a power pole outside his Rockingham home, driving a fake armoured vehicle past his electorate office, sending a package of powder to his wife, threatening to "sniper attack" his family and to "hunt them down after I am out of office".
"These things Mr Palmer does, contribute to and promote things … that I have not seen in my lifetime," he said.
"He arouses this anger and madness in our community.
"It goes on and on. This kind of crazed language and behaviour that gives people license to let loose the darker angels of their nature."
WA border closure 'not necessary'
Mr McGowan denied lying about having medical advice that said it was "necessary" to close the state's border in April 2020.
It came after Mr Palmer's lawyer, Peter Gray SC, read an email to the court, sent from WA Chief Health Officer [CHO] Andy Robertson and Communicable Disease Control Directorate director Paul Armstrong to Mr McGowan on March 29.
It stated Dr Robertson and Dr Armstrong agreed "that closing the border will have the effect of slowing the spread of COVID".
But it went on to state it "may not reduce the risk significantly further than that which is achieved by measures already in place".
Mr Gray read another portion of the email saying that closing the border may have a similar risk reduction to shutting down retail shopping.
It said if the border was put in place, it would have a "relatively small impact in relation to all other measures taken to date".
'We did what we needed to do': McGowan
Under cross-examination, Mr Gray put it to Mr McGowan that the health advice did not say, either explicitly or in a different phrasing, that closing the border was necessary.
"I do not agree with that," Mr McGowan said.
"We were dealing with a deadly virus … we did what we needed to do."
Mr McGowan said his understanding was that it was necessary to close the border because it would slow the spread of the virus.
"Isn't it plain Mr McGowan, that Dr Armstrong and Dr Robertson were simply pointing to factors to consider when you take a decision as to whether to close the border? Mr Gray asked.
Mr McGowan replied: "They were giving us advice as to what would work."
Mr Gray: "They were leaving the decision to you?
Mr McGowan: "I am the Premier."
Mr Gray said what Mr McGowan told the public was that the medical advice stated closing the border was "necessary".
"That was not true, was it?" Mr Gray asked.
"I disagree with that statement," Mr McGowan said.
Mr Gray also quizzed the Premier on the CHO's comments in September and October that WA could open to states without community transmission, using a "hot spot" definition.
Mr McGowan said he recalled there being a discussion on different models that could be used but they kept coming back to a cautious approach being the most appropriate.
Palmer's comments 'unhelpful, hurtful'
Mr McGowan also told the court comments that he had lied about about the health advice at the start of the pandemic left him "angry, hurt, offended and exasperated".
He said 2020, when coronavirus entered the state, was a dramatic and frightening time of high anxiety and terror.
"It was a very difficult and demanding period of my life.
"That someone in that context [would suggest] that I was a liar in what was basically the biggest decision of my life … that cuts to the core of what I stand for."
Premier quizzed over hydroxychloroquine
He was also grilled on his comments around Mr Palmer's desire to enter WA in 2020 for the purpose of "progressing" a donation of hydroxychloroquine to the federal government.
The court heard Mr Palmer had purchased 33 million doses of the drug in 2020 to add to Australia's national stockpile after comments by the Federal Health Minister in an April interview, which indicated it had potential as a treatment.
Mr Gray said Mr Palmer was authorised by the federal government to purchase the drug for that purpose.
He said when Mr Palmer was applying to come to WA to progress this in May, the view that hydroxychloroquine was not an appropriate treatment COVID, and indeed may be harmful, had not yet emerged.
But in an August 3 press conference, Mr McGowan made comments that were "obviously damaging" to Mr Palmer about this venture.
"What you said was when Mr Palmer sought to come to WA back in May he was seeking to do so to promote a drug that on all of the evidence was dangerous," Mr Gray put to Mr McGowan.
Mr McGowan agreed that was the case and said he could not recall precisely when he became aware of the risks of hydroxychloroquine.
But he said he had sought advice from both the director-general of the Department of Health and the state's chief health officer and both "shuddered and recoiled" and advised him it could cause heart arrhythmia and other conditions.
McGowan concedes comments unhelpful
He was also asked about a series of comments he made about Mr Palmer, including that he was a "menace" and telling him to "stop being a jerk".
Mr Gray pointed to the front page of the WA's daily tabloid newspaper, which portrayed Mr McGowan as "Mr 89 per cent" and Mr Palmer as "Dennis the Menace", published days after his comments.
Mr Gray characterised the comments as "vulgar name calling" and said Mr McGowan was setting out to denigrate Mr Palmer.
He also suggested he had made the comments because he thought it would be to his political advantage, which Mr McGowan denied.
The Premier argued he had been trying to persuade Mr Palmer to desist from legal action against the state, but conceded his language was "unhelpful" and not the type he would normally use.
Mr Gray also suggested Mr McGowan's characterisation of Mr Palmer as an "enemy of the state" was a reference to the Soviet Russian phrase "enemy of the people" that sought to annihilate someone as a human being, something Mr McGowan denied was in his mind at the time.
