Harold Shipman could have been driven by an "addiction to killing" when he claimed the lives of at least 215 of his patients, according to the official inquiry into his crimes.
A report by High Court judge Dame Janet Smith compiled after a year of hearings at Manchester Town Hall, showed no evidence of a motive for the GP's serial killing.
But his addiction to the painkiller pethidine in the 1970s probably led to other forms of addictive behaviour, says Dame Janet.
She says Shipman's motivation for killing will probably never be known.
"Only he can answer that question and at the moment it seems very unlikely he will," she said. "I don't think it will be possible to discover his motive without either an admission from him or an in-depth psychiatric examination, which without his co-operation is impossible."
Medical authorities and politicians are now calling for a review of the measures that should have detected Shipman's killing spree.
In Hyde, Greater Manchester, where Shipman practised from 1975 until his arrest in 1998, there was anger as the true scale of his crimes was revealed.
Father Denis Maher, 62, of St Paul's Church, Hyde, said: "I am very concerned with all the new range of feelings that have just opened up in people of anger and shock and grief.
"To see it in black and white from an official inquiry that your loved one was murdered by this man is gutting."
Dame Janet says Phase Two of her inquiry in the autumn will examine what went wrong and how the system could be improved.