A pest, a bothersome person or an animal
This term, stemming from the Latin word vermis, meaning a worm, is defined in relation to human activities, thus the animal species included within it vary from area to area.
Why this word?
Vermin is another word in the class of words referred to as American English, sometimes you’ll meet it as vermin, in others as varmit or varmint.
Vermin refers to the disease carrying rodents and insect but also applies to small predators, even such as rabbits, as they consume resources humans consider as theirs. Moreover, this term is an extremely pejorative one referring to specific class or groups of people who are considered inferior or subhuman and are often thought of as human parasites.
How to use vermin?
This word traveled around the world and acquired new meanings with each and every place it reached, for this reason the definition to it is slightly ambiguous. It is mainly, however, found referring to groups of people whose ideas and thought are considered to be disease like, groups existing outside the sociological balance of common society.
Neo-Nazis, for example, are vermin groups and so are radical religious sects whose beliefs are thought to be harmful by most of society.
Vermin can also of course relate to any person in your class or office who believes everything rightfully belongs to him and whose actions are guided by his poisonous and malevolent thoughts and ideas.