Answer by taking a quote from the following article:

Harold Frederick Shipman (14 January 1946 - 13 January 2004) was a British general practitioner and one of the most prolific serial killers in recorded history. On 31 January 2000, a jury found Shipman guilty of fifteen murders for killing patients under his care. He was sentenced to life imprisonment with the recommendation that he never be released. The Shipman Inquiry, a two-year-long investigation of all deaths certified by Shipman, which was chaired by Dame Janet Smith, examined Shipman's crimes.

Shipman's trial began at Preston Crown Court on 5 October 1999. Shipman was charged with the murders of Marie West, Irene Turner, Lizzie Adams, Jean Lilley, Ivy Lomas, Muriel Grimshaw, Marie Quinn, Kathleen Wagstaff, Bianka Pomfret, Norah Nuttall, Pamela Hillier, Maureen Ward, Winifred Mellor, Joan Melia and Kathleen Grundy by lethal injections of diamorphine, all between 1995 and 1998. His legal representatives tried, but failed, to have the Grundy case, where a clear motive was alleged, tried separately from the others, where no motive was apparent.  On 31 January 2000, after six days of deliberation, the jury found Shipman guilty of 15 counts of murder and one count of forgery. Mr Justice Forbes subsequently sentenced Shipman to life imprisonment on all 15 counts of murder, with a recommendation that he never be released, to be served concurrently with a sentence of four years for forging Grundy's will. On 11 February 2000, ten days after his conviction, the General Medical Council formally struck Shipman off its register. Two years later, Home Secretary David Blunkett confirmed the judge's whole life tariff, just months before British government ministers lost their power to set minimum terms for prisoners.  While many additional charges could have been brought, authorities concluded that a fair hearing would be impossible in view of the enormous publicity surrounding the original trial. Furthermore, the 15 life sentences already handed down rendered further litigation unnecessary.  Shipman consistently denied his guilt, disputing the scientific evidence against him. He never made any public statements about his actions. Shipman's wife, Primrose, steadfastly maintained her husband's innocence, even after his conviction.  Shipman is the only doctor in the history of British medicine found guilty of murdering his patients. John Bodkin Adams was charged in 1957 with murdering a patient, amid rumours he had killed dozens more over a ten-year period and "possibly provided the role model for Shipman". However, he was acquitted. Historian Pamela Cullen has argued that because of Adams' acquittal, there was no impetus to examine the flaws in the British system until the Shipman case.

Why was he tried?
Shipman was charged with the murders of