This trope refers to minor characters that are killed off to show how a monster works.

The answer is "Red Shirt" because this trope refers to minor characters who are killed off in order to demonstrate the abilities or dangers of a monster or antagonist. The term "Red Shirt" originated from the Star Trek series, where it became a running joke that expendable crew members wearing red shirts were often the first to be killed off during an away mission. This trope has since been widely used in various forms of media to swiftly establish the threat level of a creature or villain by sacrificing these disposable characters.