paraphrased variant: 
We should not be simply fighting evil in the name of good, but struggling against the certainties of people who claim always to know where good and evil are to be found. 
Source: Hope and Memory: Reflections on the Twentieth Century (2003), Ch. 5 : The Past in the Present, p. 195
                                    
“We should not be simply fighting evil in the name of good, but struggling against the certainties of people who claim always to know where good and evil are to be found.”
Hope and Memory: Reflections on the Twentieth Century (2003)
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Tzvetan Todorov 9
Bulgarian historian, philosopher, structuralist literary cr… 1939–2017Related quotes
                                        
                                        Vol. I, p. 12 
1980s, Letters to the Schools (1981, 1985)
                                    
“This is a struggle of good and evil. And we're the good.”
His opinion of the difference between Republicans and Democrats. Remarks at a Democratic fundraiser at the home of John and Nancy Hiebert, February 25, 2005, in Lawrence, Kansas. Quoted in "Dean Roars Into Town" http://www2.ljworld.com/news/2005/feb/26/dean_roars_into/ by Joel Mathis, Lawrence Journal-World, February 26, 2005. Retrieved May 9, 2016.
                                        
                                        “Welt muss mehr denn je diese Botschaft hören,” Giessener Allgemeine Zeitung, Giessen, Germany, April 12, 2005. 
Attributed
                                    
How the Jews Created the Comic Book Industry Part I: The Golden Age (1933-1955) Reform Judaism http://reformjudaismmag.net/03fall/comics.shtml (2003)