Mada 'Adam' Kabobo was taken into custody shortly after the random attack on a street in the outskirts of Milan last May, in which three passersby lost their lives.
He told investigators that he heard voices in his head during the murder spree and was later diagnosed with schizophrenia.
Marco Scaglione, a forensic doctor, told a Milan court that Kabobo’s mental health was “not compatible with prison” following a recent attempt to strangle his cellmate at San Vittore prison, an attack Kabobo said was also prompted by hearing voices, RaiNews reported.
A judge previously ruled that Kabobo knew what he was doing during the Milan killings, despite the diagnosis.
A 64-year-old pensioner walking his dog, a 21-year-old delivering newspapers and an unemployed man, 40, died in the attack.
Footage broadcast on Italian television appeared to show Kabobo roaming the streets with the pickaxe slung over his shoulder.
Kabobo, whose fast-track trial for murder is due to begin on February 6th arrived in Italy illegally in 2011 and even though his request for asylum was refused, he was not deported.
He had been arrested several times for alleged offences including theft and vandalism.
The forensic report will be discussed by a Milan court on January 27th, when a decision will be made on whether to transfer Kabobo to a psychiatric ward ahead of the trial.