At first I thought the argument they had was about Jimmy's younger brother, Joe, but now I believe that Ethel had told Jimmy she was leaving him.
Not for his younger brother, but because she was dissatisfied with the way she was having to live, without a proper roof over her head and enough food for her and their young son to eat.
"That night my wife and I had a word or two about cooking and something or another about the camp. With that I said, 'I suppose I am in this world alone with no one to care for me. I thought you was my wife.' She said, 'Go to the devil ...' I thought I may as well die, so the Mawbey murders were committed."
Jackie Underwood claimed that when they returned to camp after the murders, Jimmy and Joe had both wanted to kill Ethel and young Sidney, and that he had prevented it.
The fact that both Jimmy and Joe wanted to do it suggests that what Ethel had done had offended both of them, not just Jimmy.
Unbeknown to Jimmy, Ethel had had a foot in both camps, his and the Mawbey's.
She had been complaining to him about the negative comments the women at the Mawbey household had been making about her being married to an Aboriginal man.
But she had also been complaining to Mrs Mawbey about Jimmy and the way she was having to live as a result of being married to him.
This latter fact was revealed by the younger sister of Mrs Mawbey after the murders.
After the five-months pregnant Ethel and Jimmy had married in December 1898, the young couple had lived in a house next door to Ethel's parents in town.
When Jimmy got the 12-month fencing contract with the Mawbey's at Breelong, Ethel's parents and siblings moved to the nearby town of Dubbo.
It must have been hard for 17-year-old Ethel, a new mother of a 12-month-old son, to no longer have her own mother for support.
On top of that, she was living in a bark lean-to beside a creek several miles from the homestead.
At least she had some female company there, even if it was not very ameniable.
It would appear that the impressionable young woman was very much influenced by both Mrs Mawbey and the school teacher, Ellen Kerz.
According to Sarah Mawbey's younger sister who was living at Breelong at the time, Mrs Mawbey had told Ethel that if she wanted a proper roof over her head, instead of a bark lean-to,she had better find another man.
It would appear that this is what Jimmy was really angry about.
But his anger towards his wife who was threatening to abandon him was even more intense.
Jimmy appears to have experienced lots of betrayals in his life, and this was the last straw.
When he was in gaol in Sydney, he used to blame Ethel for making him kill the Mawbeys and the governess at Breelong.
Despite her youth, Ethel was not a passive victim of circumstances.
She could be spiteful and vindictive, and she gave Jimmy a hard time when he would not stand up to the women at the house in her defence.
At the time of the Mawbey murders, many people thought she was to blame, but she appears to have since been exonerated.
In my view, she was definitely a contributing factor.
But the true story of Jimmy Governor is not all black and white.