Sherry Moore battered 64-year-old neighbour Sue Atkinson in her garden at The Ryedales in Hull after her bipolar disorder led her to believe she was under threat from people "out to get her".
Moore, 55, had been admitted to a psychiatric hospital more than 12 times following her diagnosis, although had not been taking her medication at the time of the attack.
Hull Crown Court heard how the victim, a retired nursery nurse, had told a friend she was concerned about Moore's "increasingly bizarre behaviour" in the days leading up to the attack on October 20 last year.
Ms Atkinson volunteered at a Hull City of Culture event that day, and was due to meet a friend in the evening but did not turn up.
Her body was found in her garden the next day by a friend who had grown concerned when Sue did not answer her messages.
Patrick Paler, prosecuting the case, told the court on Tuesday: "She saw the deceased lying in the garden with a bird bath on top of her", adding that a phone, broken sunglasses, blood and a metal pole were seen nearby.
A post-mortem examination found that Mrs Atkinson died as a result of blunt force trauma and suffered more than 100 injuries to her head, neck and chest.
Police were called and found Moore sitting on her sofa in her own home.
He said she added: "I pushed something heavy, it fell on her, I didn't think it would make her die."
The barrister said Moore was exhibiting symptoms of psychosis at the time of her arrest.
He said: "She had consistently reported that she believed she was under threat and unknown people were out to get her. She clearly was not acting rationally.
"She was suffering from an abnormality of mental function, on the balance of probabilities she acted on her paranoid beliefs."
Moore pleaded guilty to manslaughter on the grounds of diminished responsibility at an earlier hearing, and a psychiatrist recommended she was given a hospital order with a release date to be approved by the secretary of state.
But Judge Paul Watson QC, the Recorder of Hull, said he was considering imposing a jail term, with a direction to initially send the defendant to hospital for treatment, and adjourned sentencing until June 6 at Sheffield Crown Court.
He told Moore, who wore her blonde hair in a plait and stood in the dock with three female hospital workers: "It's clear to me that I need to consider all of the options and not just the most obvious one in this case.
"Those options include whether or not I should pass a prison sentence but to direct that, at least at first, you be taken to a hospital."