These tropes (evil? PURE EVIL! Good? PARAGON OF VIRTUE!, et al.) come about because ambiguous characters often confuse readers, and also because fiction has historically been a vehicle for social engineering. This latter part paints a larger than life character that the reader is supposed to aspire to be like, and thus alter their behaviors accordingly. This practice has had a rather nasty impact on believable fiction as a consequence.
Some of the things that good fiction needs are:
Actually believable characters that the reader can relate to. (This poses a significant problem when writing science fiction, because aliens need to be sufficiently human like in their thoughts and actions that humans can relate to them if they are anything other than an antagonist.) There needs to be an engaging/interesting situation happening to keep the reader's interest. (A story about joe anyman going to burger king, ordering a big mac and a large soda, sitting there and eating it with nothing eventful happening, is BORING.)
Larger than life characters, be they super heroes, super villains, angels, demons or gods; are all the way they are to make a radically larger than life plot conflict believable-- with the plot conflict being what is interesting to the reader.
You find much more human characters in other kinds of fiction where radically larger than life plot arcs dont happen. Take for instance, mystery books.