The world will not be destroyed by those who do evil, but by those who watch them without doing anything ― Albert Einstein

Source: Reddit

There are quotes by others of similar vein:

He sat back in his chair, watching her. “Are you afraid?”
“No. I always thought I would be, but I’m not.”
“Where’s that spirit of self-preservation they talk so much about?”
“I don’t know. You don’t get too excited when you feel things are logical. This is logical. Nothing else but this could have happened from the way we lived.”
“We haven’t been too bad, have we?”
“No, nor enormously good. I supposed that’s the trouble – we haven’t been very much of anything except us, while a big part of the world was busy being lots of quite awful things.”

— Ray Bradbury, “The Last Night of the World”

“The only thing necessary for the triumph of evil is for good men to do nothing.”

— Edmund Burke