助纣为虐

zhù zhòu wéi nüè

lit. helping tyrant Zhou 商紂王|商纣王[Shang1 Zhou4 wang2] in his oppression (idiom)

fig. to take the side of the evildoer; giving succor to the enemy

zhù

to help

zhòu

crupper (harness strap running over a horse's hindquarters and under its tail)

wéi

variant of 為|为[wei2]

nüè

(bound form) brutal; oppressive; harsh