Question: why do snitches get called rats?
Answer: By 1859 John Camden Hotten's Slang Dictionary would define a rat as “a sneak, an informer, a turncoat,” and by the 1950s this meaning of rat was firmly entrenched in pop culture. ... Before calling someone a “rat” meant calling them an informant, it signified a drunkard, a cheating husband, or a pirate.