John Delahaye, a type 1 diabetic, had been on remand at HMP Birmingham after three people were hurt during a service at New Jerusalem Apostolic Church in Aston in September.
A jury heard how the 46-year-old from Ettington was pronounced dead on 5th March this year after being discovered laid down in his cell close to an empty insulin pen.
He was due to stand trial later that month.
Dr Simon Trotter, a consultant pathologist, told Birmingham Coroner's Court he put Mr Delahaye's death down to a severe lack of glucose and hypoglycaemia.
He was quoted by the BBC as saying: "Given he had a pen before being locked up and it was empty when he was found, I consider this was due to an insulin overdose."
The court was told how Mr Delahaye had been allowed to handle an insulin pan despite previously overdosing at HMP Birmingham on New Year's Eve 2017.
He had been charged with attempted murder, making threats to kill, wounding, and two counts of possessing a bladed weapon.
Adam Brooks (pictured above), who was injured in the attack at New Jerusalem Apostolic Church, told Premier earlier this year how he had mixed emotions when he learned of Delahaye's death.
The hearing continues.
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