It's because when people think of evil... they think of something akin to "retards" like in this poster:
It's a WW2 propaganda poster of a drooling mad-driven black gorilla with a imperialist German helmet, depicated as inferior and stupid. There is a young fair-skinned blond in blue dress in his right arm. He is holding a club engraved with a word I can't read. Caption above the gorilla is "Destroy this Mad Brute" in big orange typeface. Below the gorilla is "U.S. Army" in black, laced ontop of an orange typeface "ENLIST." The background looks grim and polluted. The gorilla is stepping on a grey shoreline labelled as "AMERICA."