Why did Harold Shipman kill so many people?

Officially, Dr. Harold Shipman murdered at least 215 of his patients - 171 women and 44 men ranging in age from 41 to 93. After a year-long public inquiry, the 2,000-page report into his 23-year murder spree was released by High Court Judge Dame Janet Smith. The records of nearly 500 patients of Shipman's who died while in his care between 1978 and 1998 were scrutinized in the investigation. Is it hard to determine a serial killer who has a job dealing with old people and that many persons believe they died of natural causes.