Berkowitz set more than 1,000 small fires before he started killing. He found them sexually arousing. And he was just a boy when he poisoned his adoptive mother's pet bird, getting a kick out of watching it die slowly. As a youth, he also tortured other small animals. Berkowitz, like others who abuse and kill animals before moving on to human victims, got an early taste of what it feels like to have power over living things.
Some people never go beyond the fantasy stage.
There is no progression to crime. But for those who make the transition to the real world, there is generally some type of stressor that pushes them over the line. It could be the end of a relationship, the loss of a job, anything that sets them off. The satisfaction from the first murder may preclude the need for any stressors to trigger the next one. Fantasy continues to play a significant role in the offender's life after the crime has been committed, affording him a sort of instant replay that allows him to relive his crimes indefinitely.