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Monster Threat Expiration may also apply. At the beginning of the episode, the monster is an unstoppable instant-killer. Later on the protagonists have a chance against it.
The "only shown in bits and pieces" that you mentioned is Monster Delay.
The random person who gets killed could be a Monster Munch and/or Victim of the Week.
Edited by KDSeconding Monster Munch for the example given.
In Monster of the Week tv shows, a lot of times, there will be a cold open where a person is going about their business, when something strange catches their attention. Then, an unknown monster (that is generally only shown in bits and pieces) attacks them. Then, cut to the intro. It’s done to show what the main characters will be dealing with for that episode.