Howard Martin used his power as a GP to "determine the lives" of three Co Durham men - Frank Moss, Stanley Weldon and Harry Gittins. The 71-year-old doctor was accused of administering fatal overdoses of morphine after police exhumed three bodies.
Detectives began investigating Mr Martin after Mr Gittins' family contacted police over their concerns about how he died, Teesside Crown Court heard.
The GP is accused of murdering Mr Moss, 59, of Eldon, near Bishop Auckland, on 14 March, 2003; Mr Weldon, 74, from Coundon Grange, near Bishop Auckland, five days later; and Mr Gittins, of Newton Aycliffe, on 22 January, 2004.
Opening the case for the Crown, Robert Smith QC told the jury: "What the prosecution say is that Dr Martin was not dealing in the interests of his patients but had chosen to terminate their lives.
"Dr Martin made the deliberate and unlawful decision to end their lives because their time had come to die."
Mr Martin, of Penmaenmawr, north Wales, has pleaded not guilty.
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