In many Western cultures, calling someone a rat has a negative connotation and can have several different meanings.
A “fink” is slang for a person who tattles on another, or turn someone into the authorities and reveals their bad or incriminating behavior.
The term “rat” is often used in the same way to describe a person who betrays another by turning them into authorities and exposing their improprieties.
So calling someone a “rat fink” is a strong way of expressing this in street slang terms.