The insane Pyramid assassin Jitterjack is one, as he mortally wounds the Anti-Hero Black Velvet.
Subverted in the case of the first Jack-in-the-Box (Jack Johnson). For decades, it was believed that he was killed in an exploding underground base on Torres Island engineered by the Underlord. However, when his son Zach Johnson led an expedition on the island, they found footage revealing that the real killer was Mister Drama, who was dying of cancer and decided to kill himself and Jack-in-the-Box in a dramatic showdown.
Willy Pete, of Empowered, is a particularly nightmarish version. His name is a military term for white phosphorous, and his powers match the name. He's capable of causing an impact as powerful as a nuclear explosion, and generating fire that reaches temperatures as hot as the sun. His favorite pastime is skull-fucking people to death as he eats them, not always in that order, and not always before they're dead. Being a fire elemental, he doesn't need to eat. He just likes to. He specifically goes after Made of Iron superhumans because anything less turns to ash at his touch before he gets any enjoyment out of it. He goes out of his way to only kill D-List heroes and villains, not because he's weak, but because it makes people underestimate him. He likes being underestimated, as it makes people think he's a pushover until it's too late (and he can make it too late pretty damn fast). He's the recurring nightmare of Thug Boy, due to killing all of his friends. In Volume 5, he kills eight and a half capes in an instant, and then proceeds to destroy most of the d10, the Superhomeys' space station. He doesn't try for an instant during all of this. He's just that powerful. It's probably fair to say he won't be getting many people to underestimate him any more after that.
In volume six, they introduced another hero killer, Deathmonger — a superscience necromancer who has enslaved legions of dead heroes. All the supers are too scared to go after him, for fear of adding to his ranks. He's still not as frightening as Willy Pete. Wasn't this supposed to be a "sexy superhero comedy"?
In the G.I. Joe Marvel comic book, a SAW Viper misunderstood orders and killed Doc, Thunder, Heavy Metal, and Crankcase. A subsequent vehicle chase led to the deaths of Breaker, Quick Kick, and Crazy Legs. Cobra Commander, who had meant for the Joes to be escorted to the border, was furious at the SAW Viper. The SAW Viper's response? "I just wasted more Joes today than your entire legions have accounted for in nine years!" Cobra Commander throws him a party in response.
Invincible: Omni-Man kills all of the Guardians of the Globe to prevent them from interfering with the Viltrumite's invasion of Earth, the only hero who survived was the Immortal thanks to his regenerative powers.
Robot attempts to kill the vast majority of the world's superheroes to help his takeover of the world go smoothly. He succeeds.
In Mighty Morphin' Power Rangers (Boom! Studios), we have Lord Drakkon, the evil version of Tommy Oliver from a universe where he stayed by Rita's side even after being freed from the Sword of Darkness. In his timeline, he killed Jason, Billy, Alpha 5, Ninjor, the Alien Rangers and the Phantom Ranger. In Mighty Morphin' Power Rangers: Shattered Grid, he added to that list, so far, with most of the Time Force Power Rangers and, most surprisingly, the heroic Tommy Oliver.
Rat-Man has Janus Valker, who kills superheroes as a job. He's extremely proficient at it, to the point the only superhero who survived Valker trying to kill him is the title character, and only because he had the sense to steal Valker's gun shortly before Valker decided to off him.
The series also has "The Punisher", a combat robot made for the purpose of killing superheroes. While it's usually dormant, when a real superhero (defined as a superhero capable of inspiring hope in the people) appears the machine activates, tracks them down, and kills them. It's so powerful it takes Chuck Norris to destroy it.
In the Sonic the Hedgehog (Archie Comics), the current Eggman (actually an alternate version of the first one), was this, managing to kill his version of Sonic and the Freedom Fighters, and when he first made his big return he conquered Mobotropolis and forced the Mobians to retreat back to Knothole, effectively resetting the series status quo to before Robotnik Prime died, though since then he's gone back and forth between how much a threat he posed. A more extreme case is the alternate timeline where Knuckles became a new Enerjak. He became so powerful that he single handily conquered Mobius and defeated every hero and villain that tried to stop him, even Sonic becoming Super Sonic wasn't enough to stop him. Pretty much the only reason why the Freedom Fighters are around in his timeline when Silver shows up there is because he let the Freedom fighters roam around for his amusement.
Durge in the Star Wars: Clone Wars comics and the television show. The most prolific slaughterer of Jedi prior to Grievous' arrival, Durge killed god knows how many named characters, came frighteningly close to murdering Anakin and Obi-Wan on several occasions, and had to be fired into the freaking sun before he finally went down.
From the Transformers franchise, we have Bludgeon, a literal Ninja Pirate Zombie Robot. (Seriously, he would have been a pirate in Transformers: Animated) In most of his appearances, he's one of the most lethal Decepticon warriors — in the Marvel G1 comics, he eventually ascended to the position of Decepticon leader, and only lost because of The Last Autobot. In the IDW comics, he beat down fan-favorite Soundwave and his casette bots after his own minions, Iguanus and Bomb-Burst, were dispatched. In the Titan UK comics, his Movieverse incarnation is one of only TWO characters to be shown killing his enemies on-panel.
The Transformers: Last Stand of the Wreckers: While Phase Sixers are theoretically this (Black Shadow's kills were high, but only showed right before he gets Worfed, and Sixshot, while threatening, never really brings the threat of main character death to the stage), Overlord stands above them. From his very first appearance both Autobots and Decepticons fear him, and when he shows up, people die, by the end of the series, he's killed or ordered the deaths of over 60 bots including Skyquake, Kick-off, Wingblazer, Rotorstorm, and Spin-out. His reappearance in The Transformers: More than Meets the Eye continues this trend, even with all of his weapons gone, he still levels the crew and is responsible for six deaths.
The Walking Dead: The Governor. Rick and his group are terrified of him, and whenever he shows up, there is a good chance people will die. He's responsible for the deaths of Axel, Billy, Lori, and Judith. In addition, he personally kills Tyreese, Hershel, Patricia, and Alice, and cuts off Rick's right hand.]
Negan marks his first appearance by bashing Glenn's skull in with a baseball bat coated in barbed wire.
, and his powers match the name. He's capable of causing an impact as powerful as a nuclear explosion, and generating fire that reaches temperatures as hot as the sun. His favorite pastime is skull-fucking people to death as he eats them, not always in that order, and not always before they're dead. Being a fire elemental, he doesn't need to eat. He just likes to. He specifically goes after Made of Iron superhumans because anything less turns to ash at his touch before he gets any enjoyment out of it. He goes out of his way to only kill D-List heroes and villains, not because he's weak, but because it makes people underestimate him. He likes being underestimated, as it makes people think he's a pushover until it's too late (and he can make it too late pretty damn fast). He's the recurring nightmare of Thug Boy, due to killing all of his friends. In Volume 5, he kills eight and a half capes in an instant, and then proceeds to destroy most of the d10, the Superhomeys' space station. He doesn't try for an instant during all of this. He's just that powerful. It's probably fair to say he won't be getting many people to underestimate him any more after that.