The characters literally combine to mean “to help (助) King Zhou (纣) do (为) cruelty (虐)”. The story is so well-known that the literal meaning has become the figurative one: to help any tyrant, not just the historical King Zhou.
The idiom is deeply rooted in the historical narrative of the fall of the Shang Dynasty. King Zhou (纣王, Zhòu Wáng) is remembered as a monstrous ruler who, along with his wicked consort Daji (妲己), indulged in extreme luxury and invented gruesome tortures for his subjects. His story serves as a foundational morality tale in Chinese culture, a warning against tyranny and corruption. Anyone—be it a court official, a general, or a sycophant—who enabled his cruelty was seen as “helping Zhou do evil.” Thus, 助纣为虐 became the ultimate shorthand for being an accomplice to tyranny. Comparison to Western Concepts: The English legal term “aiding and abetting” is a close functional equivalent. However, 助纣为虐 is far more than a legal term; it is a profound moral and historical condemnation. While “aiding and abetting” can apply to any crime, 助纣为虐 specifically evokes the image of siding with a powerful, systematic evil, like a corrupt regime or an oppressive corporation. It has the historical weight and moral outrage of calling someone a “Quisling” (a traitor who collaborates with an enemy occupier), but applied to any powerful villain, not just a foreign one. The idiom reinforces core cultural values like righteousness (义, yì) and the responsibility to stand against injustice. To 助纣为虐 is to fundamentally betray these principles for personal gain or out of cowardice.
This is a very strong, formal, and literary idiom. You would not use it to describe helping a friend with a mischievous prank. It is reserved for serious accusations of complicity in significant wrongdoing.