The man nicknamed the "Brighton cat killer" died of COVID on his 55th birthday while receiving end-of-life cancer care, according to an inquest.
Stephen Bouquet was jailed for more than five years in July 2021 after stabbing and killing nine cats and injuring seven more.
He carried out the attacks between October 2018 and May 2019 before being caught on CCTV and arrested.
The Royal Navy veteran and former security guard died in Medway Maritime Hospital on 5 January 2022 - just six months in to his prison sentence.
An inquest has now found he died of COVID-19 pneumonitis, with a secondary cause of chronic obstructive pulmonary disease.
The Maidstone hearing also heard Bouquet was being treated for thyroid cancer while locked up at HMP Elmley in Kent.
The coroner Patricia Harding said it was "not clear" where Bouquet contracted coronavirus - either in hospital or in prison.
But he tested positive for the illness on 28 December 2021, a day after he was admitted to Medway Maritime Hospital for a cough and difficulty breathing.
Bouquet was given antibiotics, oxygen and other medication but "progressively deteriorated", said the coroner.
"Despite whatever measures, he was too weak to fight the infection and sadly he died on January 5, 2022, on his 55th birthday," she added.
'Vulnerable to infection'
The coroner said that when he was transferred to the jail in December he was already "extremely frail" and "vulnerable to infection".
However, she said she was satisfied the prison had followed pandemic guidelines.
Bouquet was convicted of 16 offences of criminal damage over the cat attacks, as well as a charge of knife possession.
His crimes led to the deaths of Hendrix, Tommy, Hannah, Alan, Nancy, Gizmo, Kyo, Ollie and Cosmo.
During Bouquet's trial, several cat owners in the East Sussex city described finding their pets bleeding on the doorstep.
He was caught after Hendrix's owners noticed a CCTV camera near where their cat had been stabbed.
The video showed Bouquet "stooping to stroke the cat before taking something from his rucksack and making a sudden jerk towards it", said Detective Inspector Chris Thompson.
A knife with cat blood on it and his DNA on the handle was found at his home.
During sentencing, Judge Jeremy Gold QC described the attacks as "cruel" and "sustained".
Bouquet previously served in the Royal Navy for 22 years, including in Northern Ireland and Iraq.