Dennis Nilsen's Psychological Profile

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Dr. James MacKeither: Claimed that Nilsen had commitment issues, maldaptive behaviors, sexual arousal with dead bodies, and narcissistic/grandiose behavior.

Dr. Patrick Gallwey: Diagnosed Nilsen with a "Borderline False Self As If Pseudo-Normal Narcissistic Personality Disorder". Essentially, Nilsen had outbreaks of schizoid disturbances that he kept hidden most of the time. Nilsen was not guilty of malice aforethought.

Dr. Paul Bowden: He found no evidence put forth by other psychiatrists. Believed Nilsen to be manipulative. He believed Nilsen to have a mental abnormality rather than a mental disorder.

Nilsen claims that he acted the way he did due to viewing his Grandfather's corpse at the age of six.
Another popular belief is that he began to act the way he did due to his roommate's departure. He was increasingly lonely ever since.