Stephen Pasceri was a licensed gun owner, without a history of violence, who strode into Dr. Davidson's clinic, asked to speak to with him, and, upon Davidson's harried appearance (in the middle of a busy office hours) proceeded to assassinate him in broad daylight. Pasceri then ended his own life shortly thereafter.
The backstory was that Dr. Davidson had recently cared for Pasceri's mother. There were complications after a procedure, she passed away. Pasceri would have probably articulated his psychotic break as an "act of retribution". Which is bullshit, of course. Several years prior to this heinous episode, he had drawn attention to himself after the death of his father regarding a bill for services rendered after his dad had been emergently transported to the hospital following a massive myocardial infarction. The old man had refused Medicare part B and the estate received a bill for 8 grand after the funeral.