I did not consider the inciting/pushing factor of such videos or news coverage. In regards to that I think that yes they would provide a push for some people either strengthening their fantasy or even a step closer to reality. This seems apparent from other contexts of deviant behavior, where fantasies are re-enforced and engrained by repetition which may lead to eventual lack of satisfaction from the fantasy alone.
However, I would suspect that unless someone is actively being treated for something and actively excluding stimuli that aggravate negative tendencies and thought patterns, that the presence of of those specific types of videos is inconsequential. Even if that specific type of media were highly controlled, there would always be an outlet for the fantasy. There has been too much violence recorded in history and those specific types of media are a mere drop in the bucket, and someone predisposed to a violent fantasy, unchecked, will seek out similar available materials.