From the criminal sphere, a series of guarantees are established in favour of the justiciable, but -equally- there is a purpose of penal protection to a society that claims protection and fight against crime. We address, in this study, the relationship of mental illness with crimes against life, emphasizing that -frequently- we confuse mental illness with dangerousness, explaining the crime based on the ailment presented by the author. It is necessary, in this sense, to delve into the possible conditions associated with the criminal act (etiology), as well as its manifestations (phenomenology), seeking prevention and treatment.