为虎作伥 to serve as a Chang ghost for the tiger
Explanation
这个成语比喻充当恶人的帮凶，指为别人做坏事，帮着别人做坏事。
This idiom means to act as an accomplice to an evildoer, i.e., to do bad things for others and help others do bad things.
Origin Story
古代，有一种传说，说被老虎咬死的人，死后会变成伥鬼。伥鬼会听从老虎的命令，为老虎引诱人来吃。老虎要出去觅食的时候，伥鬼就会走在前面，帮助老虎躲避陷阱，找到人之后，伥鬼就会上去抓住那人，把他的衣服脱掉，让老虎吃掉。老虎吃饱了之后，新的伥鬼就会产生，旧的伥鬼就可以自由活动了。人们就把这种行为叫做“为虎作伥”。
In ancient times, there was a legend that said that people who were bitten to death by tigers would become Chang ghosts after death. Chang ghosts would obey the tiger's orders and lure people for the tiger to eat. When the tiger went out to hunt, the Chang ghosts would walk in front, helping the tiger to avoid traps. Once they found a person, the Chang ghosts would go up and grab the person, take off his clothes, and let the tiger eat them. After the tiger was full, a new Chang ghost would be born, and the old one would be free to move around. People called this behavior "serving as a Chang ghost for the tiger".
Usage
该成语用来形容那些为了自己的利益而帮助坏人做事的人，或是指那些明知错误却依然支持错误行为的人。它带有强烈贬义，常用于批评、讽刺那些助纣为虐、为虎作伥的人。
The idiom is used to describe those who help bad people do things for their own interests, or to describe those who are aware of the wrong but still support the wrong behavior. It has a strong derogatory connotation and is often used to criticize and ridicule those who help the tyrant and serve as accomplices to evildoers.
Examples
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一些人为了个人利益，不惜～，帮助坏人做坏事。
Yi xie ren wei le ge ren li yi, bu xi wei hu zuo chang, bang zhu huai ren zuo huai shi.
Some people are willing to help bad people do bad things for their own interests.
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他明明知道这个计划不可行，却依然～，为虎作伥。
Ta ming ming zhi dao zhe ge ji hua bu ke xing, que yi ran wei hu zuo chang, wei hu zuo chang.
He knew the plan wouldn't work, but he still helped it.
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我们一定要坚决抵制腐败现象，不能～，助纣为虐。
Wo men yi ding yao jian jue di zhi fu bai xian xiang, bu neng wei hu zuo chang, zhu zhou wei nue.
We must resolutely resist corruption, we must not help the tyrant.
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他为了钱财，不惜～，为坏人提供帮助，最终也受到了法律的制裁。
Ta wei le qian cai, bu xi wei hu zuo chang, wei huai ren ti gong bang zhu, zui zhong ye shou dao le fa lv de zhi cai.
He was willing to help others for money, and was eventually punished by the law.