Black Eyes of a Psychopath

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I've been trying to find out how a psychopath's eyes can turn black - like one would imagine a demon would look. Can anyone's eyes do that? After over a decade with my psychopath ex-SO one day I saw those black eyes and his face looked absolutely, unmistakably evil. It was terrifying, like a totally different person. It lasted maybe a couple of minutes at the most, and I only saw it once but I can't forget it. Can an expert please explain that phenomenon? Did he do it on purpose? He was very calm.