Helping the oppressor:
The Messenger of God(SAW), said: 'Help your brother, whether he is doing wrong to others,or is being wronged.'A man protested: ' O Messenger of God,(SAW),I can understand helping one who is being wronged, but how can I help when he is doing wrong?'The Prophet (SAW) replied: 'Stop him from doing wrong : this is your help to him.'
(Bukhari,Muslim:Anas)