During the night of the 11 to 12 of December 2002, Mathieu X. 21 years old, convinced he was defending himself from evil human beings decapitated a nurse and an auxiliary nurse of the psychiatrist hospital. This crime, which received saturated media coverage, obviously raises questions about the dangerous and violent nature of the mentally ill, which can sometimes culminate in homicide. Firmly rooted in the collective consciousness is the popular idea that someone who kills an unknown person in the street is mentally ill. Conversely, the epidemiological data are reassuring; only 15% of such crimes are committed by the seriously mentally ill (schizophrenia, paranoia, melancholia).Typing and comparison of homicides committed by schizophrenic, paranoiac and melancholic persons.Several murders committed by psychotic persons are presented in this article. This retrospective study shows several types of pathological murder (schizophrenia, paranoiac delirious disorder, affective disorder: melancholia and hypomania). Twenty-seven cases have been selected and analysed from 268 cases prepared over 30 years by two psychiatrists, whose diagnoses were schizophrenia, paranoia, melancholia or hypomania.From these 268 cases of homicide examined, 27 murderers were psychotic. Ten of these were young, single, jobless, male schizophrenics: they drank little alcohol. Most of them had a criminal history. They were paranoid schizophrenics whose hallucinatory mechanisms fed mostly persecuted, sexual and metaphysical themes. Forty percent of them were disorganised, and half of them showed negative features. They knew their victim (family, friends). Nine others were paranoiac, for the most part male, older, married, family men, without psychiatric or criminal record. Intuitions with delirious fed persecuted (77%), jealous (40%) or prejudicial themes. They murdered their wife or husband or neighbour. Alcohol consumption was often involved. Schizophrenic and paranoiac murderers often have an emotional temper. Conversely, melancholic murderers are mostly female aged around 30, married, family women, drinking little alcohol. Two-thirds of them have psychiatric records of depression, bipolar disorders and attempted suicide. Altruism is the most frequent delirious theme. Their murders are more often premeditated. They know the victim: child or partner. Suicide often follows the murder.