Former security guard Stephen Bouquet, 55, died in hospital after being jailed for five years in 2021.
Bouquet carried out a string of violent attacks on cats in Brighton between October 2018 and May 2019, killing nine and injuring seven more.
At an inquest into his death in Maidstone, coroner Patricia Harding said: “Despite whatever measures, he was too weak to fight the infection and sadly he died on January 5 2022, on his 55th birthday.”
The coroner also noted that earlier in December when Bouquet was transferred to HMP Elmley, he was “extremely frail”, had poor mobility and was “vulnerable to infection”.
But Ms Harding also said she was “satisfied” that the prison followed national guidance during the pandemic to minimise the spread of the global virus.
Bouquet was receiving end-of-life care for thyroid cancer while serving his sentence.
The coroner said it was “not clear” where Bouquet contracted coronavirus, either in hospital or in prison, but he tested positive for the illness on December 28, a day after he was admitted to Medway Maritime Hospital for a cough and difficulty breathing.
Bouquet was serving his five-year-sentence after being found guilty of 16 offences of criminal damage in relation to cats, as well as possession of a knife.
He carried out a string of attacks on pets in Brighton between October 2018 and May 2019, before being caught on CCTV set up by one of the victim’s owners.
Nine cats – Hendrix, Tommy, Hannah, Alan, Nancy, Gizmo, Kyo, Ollie and Cosmo – were killed, while another seven were injured.
Bouquet had previously served in the Royal Navy for 22 years, including in Northern Ireland and Iraq.
The coroner ruled that Bouquet died of natural causes with the medical cause of death being Covid-19 pneumonitis.
Ms Harding gave her condolences to Bouquet’s brother who attended the inquest.
She added: “Covid not only had an effect on everybody touched by it but also its lasting effects (mean) the investigation has taken a lot longer.”
“I’m sorry it’s taken so long.”