Superman is often considered not just the most powerful superhero, but the most pure.
Even though the Boy Scout can crush a diamond with his bare hands or lift a planet, Superman's greatest strength comes from his compassion towards all living things.
Watching the Man of Steel saving a cat from a tree, reading stories to children in a cancer ward or comforting a teenager battling depression is just as important (if not more so) than watching him capturing the Toyman or bashing an alien in the face repeatedly until the world is safe.
That's why it's so jarring to see Superman depicted any other way besides noble and altruistic. Through cloning, experimentation, or brainwashing, the Metropolis Marvel has been forced on many occasions to face an evil incarnation of himself. Some of these doppelgängers are from parallel worlds where the word" Superman" is associated with mass-murder, genocide, or Nazism.
When you witness a supervillain bearing Kal-El's face take over the world, obliterate cities with a single punch, or gleefully slaughter his friends and fellow superheroes, it makes you realise just how terrifying an evil Superman can be, and why it's a trope writers have repeatedly fallen back on in recent years.
10. Bizarro - Superboy #68 (1958) + Action Comics #254 (1959)
Not only was Bizarro the first evil version of Superman, he popularised the trope of a hero having a wicked doppelgänger.
In Superboy #68, a teenage Clark Kent got shot with a duplicatory ray, which created a double of him called Bizarro. This creature perished at the end of the issue but fans were won over by the character, which encouraged DC's creators to bring Bizarro back one year later.
His backstory has been retconned many times over the years but Bizarro is usually referred to as a clone of Superman created by Lex Luthor. Because the cloning process was flawed, the disfigured ersatz has impaired intelligence and the inverted powers of Superman. He also speaks in opposites, so he'll yell "Goodbye" as a greeting and "I love you" when pummelling his enemies.
Bizarro is as much a victim as he is a villain, but there's no getting away from the fact he's done some pretty evil things over the years.
Because of his impaired intelligence you'd probably think it'd be easy for the Last Son of Krypton to wipe the floor with him, but since he is Superman's opposite, Bizarro has one major advantage over his counterpart; a complete immunity to Green Krytponite. In fact, kryptonite has been shown to boost Bizarro's intelligence, giving the Thing of Steel an edge over his better half.