为虎作伥（爲虎作倀）　

◎ 为虎作伥 wèihǔ-zuòchāng

[help a villain do evil;play the jackal to the lion;hold a candle to the devil] 为老虎引路的鬼。比喻给坏人做帮凶,为坏人效劳。亦作“为虎添翼”