There are some references to God in To Kill a Mockingbird, but few if any to an evil supernatural source such as Satan. At no time does Scout blame human cruelty on anything other than the humans that perpetrate it. In this way the novel connects with Conrad’s quotation.
We see wickedness in several forms in Maycomb. Bob Ewell is very nearly evil incarnate with his hateful attitude toward Tom Robinson and Atticus Finch. The men that meet at the jail to demand Tom Robinson are an example of how evil infects a mob. Mrs. Dubose shows how drug addiction can make a person behave in a hateful manner.
Atticus attempts to explain human wickedness to Scout in terms of the poverty and ignorance that some people live with. To a certain extent he is resigned to the presence of human evil, and believes that the best a person can do is be a good example to others.