What do you call a person that wants to kill people but doesn't actually kill them?

Homicidal ideation is a term describing the mental disorder such a person suffers from. I don't know of a noun describing the person. homicidal ideator doesn't seem to be a thing. According to Wikipedia:

"Homicidal ideation is a common medical term for thoughts about homicide."1

According to lecture notes (for preparing to be an emergency nurse) on Study.com:

"Homicidal ideation is a thought pattern that may lead to actual homicide or the killing of one person by another. In this lesson, we will discuss the assessment of a person's risk for committing homicide, and related causes and management."2

Attribution

1 Homicidal ideation. (2018, March 12). Retrieved March 22, 2018, from https://en.wikipedia.org/wiki/Homicidal_ideation

2 Jankowski, P. (n.d.). Homicidal Ideation: Definition, Assessment & Management. Retrieved March 22, 2018, from https://study.com/academy/lesson/homicidal-ideation-definition-assessment-management.html