But I'm not surprised one bit that during the development of Atticus Finch as a character in literature, there was moral ambiguity.
White people in Alabama of that day can be put into several categories. The overwhelming majority of them supported segregation. By "overwhelming" I mean 90%, roughly the same percentage of white people in Alabama who voted for the opponents of Barack Obama in 2008 and 2012. Of the remaining 10%, nine of ten were on the fence about segregation; they couldn't fully embrace its immorality, yet they were paralyzed by indecision and inaction that are pitfalls of pragmatism. This leaves one in a hundred white people in Alabama who thought segregation was wrong and were prepared to do something about it. Fortunately the 1%, among them Morris Dees, Richmond Flowers, and Frank Johnson, had a disproportionate influence on events. But they were still only 1%.
Mockingbird, the Hollywood production of it, and in particular Gregory Peck's portrayal of Atticus Finch led to the secular canonization of the character as an example of what white people in the South could and should be. Hagiography is very important, but I give thanks that it's not the only genre. For every Atticus Finch there's a Jack McCoy. We identify with moral ambiguity because it's where more of us are most of the time.
In my calculus of Alabama, the Atticus Finch of Watchman is the 9%; St. Atticus of Mockingbird is the 1%. Both are historically accurate. If that comes as a shock to you, I'm happy that Lee has challenged your misinterpretation of southern sociology — whether she meant to, or not.