Why is rat an insult?

Calling someone a "rat" is often used as an insult because it evokes negative associations with the animal, which is commonly seen as dirty, sneaky, and untrustworthy. In many contexts, rats are associated with betrayal or informing, particularly in criminal or gang cultures where "ratting" refers to someone who betrays others by revealing secrets. This metaphorical use implies that the person lacks loyalty or integrity, making it a powerful derogatory term.