After being jailed for two years and taken down to the cells, he was then found in a distressed state.
He told staff he had taken the poison strychnine, for which there is no antidote.
Despite desperate attempts to revive the 48-year-old, he was pronounced dead an hour later in hospital.
The inquest heard that in the days leading up to the case on August 7, 2008, Mr Key had told friends and family he "could not go to prison" and had become increasingly distressed.
His wife, Rachel, told the inquest she had seen him visibly upset the night before the sentencing but did not believe her husband would take his own life.
She said: "He said that he knew what he was going to do. I thought he was just getting it all out of his system. I was a bit more positive and said we just had to try and think positive.
"I did not think he would leave the kids, I did not think he would go through with it."
Family friend John Ayre described Mr Key as an "out-and-out country man" who was "loved by everyone".
He told the jury how he watched his friend become "quieter and sadder" in the weeks before the court date as he was concerned about being handed a prison sentence.
Mr Ayre said: "A custodial sentence to him would be like putting a lion in a bird cage. He was a country, outdoors person. None of us knew what he was going to do, which is tragic."
The hearing was told Mr Key, who had no previous convictions, was initially arrested in February 2008 after police and RSPCA staff executed a warrant at his home at Alkmonton, near Ashbourne – acting on intelligence over suspected badger baiting.
While carrying out a search, officers discovered two unlicensed firearms. No charges were brought against Mr Key in relation to the suspected badger baiting but he was charged with firearms offences.
Three bottles of strychnine, a banned poison commonly used in pest control, were also found during the search.
The jury was told how police consulted English Nature about what to do with the poison. They were advised to leave the bottles and an enforcement officer would contact Mr Key to help arrange their disposal.
During the inquest yesterday, Mr Key's wife and his son, Ben, said they believed all the strychnine had been disposed of correctly and that there was none left on the premises.
But a blood sample taken from Mr Key after his death showed a "toxic" level of the poison in his system.
A blue plastic cup and blue "wrap" found in his cell also showed traces of the poison, which Mr Key was known to have used in his pest control work.
The inquest is due to continue today