Siding with the bully
助纣为虐
zhùzhòuwéinüè

It literally means "aid King Zhou in his tyrannical rule."

The Story:
In the late Qin dynasty, after capturing the capital of Qin, Liu Bang saw the magnificent, beautiful palace of Qin. In there was so much gold and treasure, not to mention young and beautiful women. He was tempted to the point that he wanted to live in the palace and pursue his desires to the very end. However, his advisor, Zhang Liang did not agree with the decision. He said, "The Qin dynasty was overthrown because its ruler was brutal and tyrannical. If you want to destroy evil for the sake of the people, then you need to discard the extravagance that characterized the Qin, and instead preach the importance of working hard and living a plain life. If you live in the palace and indulge on your fantasies, then you are the equal of King Jie, who was the last king of the Xia dynasty. An effective medicine tastes bitter but prevents sickness, and words of wisdom sound shrill to the ear but is beneficial for upright behavior." When hearing what Zhang Liang's comment, Liu Bang finally saw the error of his ways. He stationed his men elsewhere, knowing that he did not want to be the equal of King Jie.