That he had disliked me at the time of my visit I was well aware; but now I could divine the maniacal fury which lay behind those vigorous heart-pulsations on which I had remarked to Le Marinel. He then, of course, had regarded me at the one man likely to thwart his homicidal purpose by getting him put under restraint, and from my experience of delusional dementia I was aware of how easily his hatred could be temporarily transferred from the original object of his hate to myself.
The workings of a disordered mind are hard to follow, but it is an error to suppose an insane person cannot conceive, and adhere to a purpose. There is method in madness; and with homicidal lunatics the doctor frequently becomes an object of intense hatred, the first enemy who must be removed before the original murderous intention can be achieved.
From John Ferguson, Death Comes To Perigord (Chapter XI: How We Caught de Quettville). London and Glasgow, William Collins and Sons, Co., Ltd., 1931.