Killing creates sin, but the amount of sin varies depending on who is being killed (animal or human), with what intention or under which circumstance
Through spiritual research we have found that any one of us can become a ghost. The desires, cravings and attachments we have during our life on Earth if not controlled and overcome, become our weak point in the afterlife. Higher level negative control lower level ghosts in the afterlife.
In horror movies we often see ghosts terrorising people. However this is not the only reason they affect people. Depending on a person’s activities the objectives of ghosts may change.