Helping the Tiger to Pounce upon Its Victims

An ancient legend has it that a tiger ate a man, and the man's soul could not be freed until it found another man for the tiger to eat. 

古代有个奇怪的传说：有一只老虎把人咬死以后，吃掉人身上的肉，却不让他的灵魂离开。这个灵魂下次还要帮着老虎去吃人，这样的人叫伥鬼。伥鬼带着老虎去寻找第二个人，让老虎再把那个人咬死、吃掉，这时伥鬼才能够获得自由。 

This idiom means to do evil things in the service of the wicked.

这个可笑的传说，后来就变成了“为虎作伥”这个成语，用来比喻替恶人做帮凶，引诱或帮助恶人干坏事。