THE murder of 8 student nurses in Chicago and the slaughter of 15 persons in Dallas share many common elements, not the least of which is the significant involvement of the medical profession. In the nurses case it was a physician who was responsible for the apprehension of Richard Speck, calling the police after he noticed a resemblance to the composite sketch of the suspected killer and the tattoo, "Born to Raise Hell." In Dallas a psychiatrist became the subject of a national discussion when he let it be known that Charles Whitman had given expression to a fantasy of . . .