How does it fit into the film as a whole?

This opening fits into the film because it gives us the two main themes of the story. One is that the watchmen are being targeted and set up by an unknown threat.  This is shown by the masked assailant breaking into the comedians apartment and murdering him. The second theme is the threat of nuclear war which is shown through a news interview where they discuss the possibility of the Russians starting a nuclear war with America . These themes set the mood of the film and stay within the story throughout the film.