Evil does not need to triumph. Evil wants only to wreak what havoc it can.
Evil is opportunistic. Evil does not need good people to do nothing: it just needs us to do less than enough.
Evil thrives on our fear and self-righteousness; on our failing to recognize it or its approaching shadow, particularly in its seemingly small forms; on our doing less, individually and as a society, than we could do to detect it, name it, and fight it; and on our confusion and quarrels about how to fight it and who should fight it.
Who should fight evil? Each of us, as our abilities permit.