"The world will not be destroyed by those who do evil, but by those who watch them without doing anything." - Einstein
It can also be paraphrased, more closer to Einstein's words as, "The world is a dangerous place not just because of those who commit evil, but because of those who silently watch."
This quote comes from what Albert Einstein said about Pablo Casals, a Spanish cellist in 1953:
It is certainly unnecessary to await my voice in acclaiming Pablo Casals as a very great artist, since all who are qualified to speak are unanimous on this subject. What I particularly admire in him is the firm stand he has taken, not only against the oppressors of his countrymen, but also against those opportunists who are always ready to compromise with the Devil. He perceives very clearly that the world is in greater peril from those who tolerate or encourage evil than from those who actually commit it.