To decide what is considered evil in the novel, first consider what Lee associates with goodness. For our shining example of good, we don't need to look any further than Atticus Finch. Atticus is nearly the perfect character-- almost christlike. He stands up for what he believes in, stays true to himself, sets an examples for others, and helps those who can't help themselves. His COURAGE, to stay true to what he believes is right no matter the pressures from the outside world, is truly astounding and we can see Jem and Scout learn from his example throughout.
In contrast to Atticus, we see the COWARDLY EVIL of Bob Ewell. Bob is crass and unintelligent where Atticus is shows grace and logic. Bob is ignorant, violent, and lazy whereas Atticus is aware, tolerant, and hard working.
The evil represented by Boo is shown in a specific example during the mob scene at Maycomb's jail. When the faceless mob gives in to the backward racism and hate, they turn to violence and ignore the laws of justice. Luckily, when Scout puts a face to the mob in Mr. Cunningham, the evil sees itself and dissipates.