Prophet of Allah (saws) has said: 
"You people must preach truth, and prevent the sinners from doing forbidden things, and check the tyrants, to bring them towards the right path, otherwise you will be accursed and your hearts will be corrupted, just as Allah did with Bani Israel."
The Prophet of Allah (saws) read certain relevant verses of the Holy Qur'an to emphasize this subject.  Bani Israel were condemned because among other things, they did not prevent others from doing forbidden things.
Explanation: 

Nowadays it is considered a virtue to be at peace with all, and to please everyone on every occasion.  They say it is a requirement of good manners with the evildoers.  Obviously, this is a wrong policy, because at most there may be some religious sanction for keeping quiet in extreme case, but never for falling in the line with the tyrants and transgressors.  At the very least, every one must instruct those people to do good whom he easily can influence.  

Sufyan Sauri says: "Whoever is very popular with his relations and neighbors  we suspect him to be compromising in preaching the true teachings of religion.