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.