Have a look at wikipedia. Homicidal ideation is not a mental illness. It is actually common, at least among university students surveyed.
Homicidal ideation is a common medical term for thoughts about homicide. There is a range of homicidal thoughts which spans from vague ideas of revenge to detailed and fully formulated plans without the act itself.[1] Most people who have homicidal ideation do not commit homicide. 50–91% of people surveyed on university grounds in various places in the United States admit to having had a homicidal fantasy.[2] Homicidal ideation is common, accounting for 10–17% of patient presentations to psychiatric facilities in the United States.[1]
Homicidal ideation is not a disease itself, but may result from other illnesses such as delirium and psychosis. Psychosis, which accounts for 89% of admissions with homicidal ideation in one US study,[3] includes substance-induced psychosis (e.g. amphetamine psychosis) and the psychoses related to schizophreniform disorder and schizophrenia. Delirium is often drug induced or secondary to general medical illness(es).
It may arise in association with personality disorders or it may occur in people who do not have any detectable illness. In fact, surveys have shown that the majority of people have had homicidal fantasies at some stage in their life.[2] Many theories have been proposed to explain this.