A killer is a person that has already killed someone. The term i'm looking for is someone who wants to kill people but never does. Only has a tendency to do so, or wants to kill them but never really does.
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
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