He goes on to describe the factors involved in compulsive behaviors, from the initial first kill, with its exploratory fumbling, to the cycle of action and need that evolves into a behavior in which the killers feel comfortable. They can also grow addicted. They get excited when they have accomplished a kill, and then if they havent been caught, they grow confident that they can do it again.
The erotic quality of the act reinforces it and turns it into a consuming obsession. The compulsion and ritual of the kill retains its importance, Giannangelo states, and the intensity level will diminish with each new incident. Thus, as with any addiction, the need grows stronger, pushing a killer to act more often and with more brutality as he tries to relive the high of the first kill. In general, he wont stop until he is stopped by some force other than his own will.
Jack Unterweger fit the profile of the compulsive killer. The police knew they had to stop him, so they turned to the acknowledged world experts for assistance.