When combined, the characters literally translate to “to commit (作) evil (恶) of many (多) types/items (端).” The key is the combination of 多 (duō) and 端 (duān), which emphasizes not only the high quantity but also the broad variety of the evil deeds.
“作恶多端” is deeply rooted in Chinese concepts of morality, justice, and karmic retribution (报应, bàoyìng). It reflects a cultural understanding that true evil is not a single act but a consistent pattern of behavior that harms society. It's a term you'll frequently encounter in wuxia (武侠) novels, historical dramas, and news reports about major criminals. A Western parallel might be a “career criminal,” “serial offender,” or a person described as “monstrous.” However, “作恶多端” carries a stronger and more formal tone of moral judgment. While “career criminal” describes a profession, “作恶多端” describes a person's fundamental moral character. It evokes the idea that such a person has violated the “Way of Heaven” (天道, tiāndào) and will inevitably face justice, either from the legal system or from fate itself.
This is a formal and very strong term. It is not used lightly in casual conversation.
You would not use this to describe a child who misbehaves or a friend who tells a lie. The actions must be serious, numerous, and varied.