1) a number of workmen or prisoner working together; a group of criminals (gang)

2) a member of a gang of armed criminals (gangster)

3) a machine used for beheading criminals with a heavy blade sliding in grooves dropped from a height (guillotine)

4) a man who hangs criminals (hangman)

5) a disorganized crowd, especially one that has gathered to make trouble or attack; a gang of criminals (mob)

6) an official group of men and women whose job is to protect people and property, catch criminals etc (police)

7) a structure put up for workers round a building which is being built or prepared; a platform on which criminals are executed (scaffold)