Doomsday. He mercilessly kills everything he encounters and takes great delight in it. Even without taking into account his messed up origin, his apparent immortality or how they eventually got rid of him, he's a scary opponent, even for Superman.
The classic "bloody S-shield". Many storylines with such a widespread change likes to invoke this feeling by altering a famous logo in some way. The Death of Superman did it first with the S-shield just bubbling and leaking blood. Very few logos can invoke such dread.
The 2018 animated Movie
Doomsday is terrifying. He's simply a feral, raging beast whose entire existence is devoted to murdering anything and everything around him, except he's as strong as Superman. His dialog consists entirely of animalistic roars, snarls, and growls, all of which manage to be blood-chilling. Worse yet is the girl sending a call for help as Doomsday is destroying everything, highlighting just how terrifying that coming after you would be.
Doomsday literally ripping a group of Atlantians to pieces with his bare hands in a matter of seconds. The sound effects do not help. He then slaughters the crew of Lex's sub as a follow up. In its own way, Lex Luthor's reaction, or lack of it, to the massacre is terrifying as well.
Early on, a couple that was out camping is menaced by a wild bear. Then Doomsday walks in and proceeds to massacre the bear, the husband, and the wife, in a sequence that feels like it was ripped straight out of a horror film.
When he's fighting Superman, Doomsday grabs the Kryptonian's cape and wraps it around its wearer's neck. This shows that despite everything, Doomsday is intelligent. He can think, he can plan; he might be a beast, but he is not a mindless one.
Doomsday preparing to kill Lois has him striding casually and quietly towards her, extending his bone spur as he does so, then stopping and standing mere inches before her, and then slowly raising his arm up to deliver the kill; all in a manner of deliberately savoring every second of this moment before he slaughters this helpless and fragile figure before him, who can only stand in place knowing any attempt to escape is futile.
The worst thing about Doomsday is how utterly unstoppable he is. Neither Hawkman nor Green Lantern manage to so much as stagger the beast; Flash doesn't hold back at all, but Doomsday overtakes him in short order. Batman is completely out of his league, Cyborg gets literally torn apart, Aquaman and Martian Manhunter are swiftly defeated. Wonder Woman holds her own for maybe thirty seconds, but the Amazon princess falls once her sword shatters against his fist. And against Superman, he's equal or greater; every time Supes seems to get an advantage, Doomsday takes back control of the fight. His never-changing expression makes every move seem effortless.
The very last shot in the film: the Cyborg Superman flying through space straight toward the Earth. Seems like an awesome moment, unless you're familiar with his intentions in the original story, which immediately causes it to have a much more menacing vibe...
