The Anti-Monitor, who personally beat Supergirl to death, after having already eaten hundreds of universes, and went on to be responsible for a scad of other hero deaths, his Shadow Demon creations would kill several more heroes during the storyline including Dove, and Kole of the Teen Titans, as well as Sunburst. He's the standard by which DCU characters judge "evil" and "dangerous". In a mostly successful attempt to avoid Villain Decay, DC only used him three (and a half) times from 1985 to 2010, a full quarter century.
Chase has Doctor Trap, who murders several Heroes and Villains for killing his wife in their battle, he killed them by using various traps tailor made for each of them.
General Zahl from Doom Patrol successfully blew up the original Doom Patrol team, however it was eventually revealed that most of them survived (albeit severely injured and barely alive), and it turned out Elasti-Woman was the only member who had truly died (Though the Chief was later able to resurrect her after Infinite Crisis).
Doomsday is the ultimate bioengineered killing machine and lived up to that feat by tearing through the JLA and killing Superman in The Death of Superman. While he's never topped that feat, he remains a deadly threat to all life, everywhere, and his entrance into a story is usually a sign that the bodycount is about to skyrocket.
DC has tried to do this with Deathstroke. For several years he was pushed as one of the top villains of the DCU, on par with Lex Luthor and The Joker. He actually has managed to kill a couple of heroes, including Phantom Lady and the third Atom (and in the trailer for DC Universe Online, he almost kills Batman). Unfortunately, while his powerset (he's basically an evil Captain America with a healing factor thrown in/Deadpool with sanity) should make him utterly terrifying to normals and low-level metas, it's a little more difficult to accept him as a serious threat to the likes of Superman and Green Lantern. His victories against big-name heroes tend to depend on his opponent firmly grasping the Idiot Ball and refusing to let it go (e.g. Flash running straight onto his sword; Green Lantern deciding to swing punches at him instead of flying off and using a ring construct from a distance; or Superman apparently forgetting how his own powers work). Fans have joked that Deathstroke's secret power is a "jobber aura" that makes his opponents behave like morons.
To rub salt in the wound, originally Slade was savvy enough to AVOID facing superhumans. He just faced the Titans to complete the contract his deceased son had accepted.
In Doomsday Clock, Doctor Manhattan killed Alan Scott by changing the past and erased the Legion of Superheroes from the future though in the end, he reversed these. In DC Universe Rebirth #1, which set the stage for Doomsday Clock, he killed Pandora by vapourizing her.
Savitar, one of the enemies of The Flash, has quite a body count to his name, having killed Cassiopeia of the Red Trinity and Johnny Quick. The Flash (Infinite Frontier) brought him back and added more by revealing his attempts to take over the Speed Force, and not Wally's breakdown, is what caused the Speed Force explosion that killed most of the heroes at Sanctuary in Heroes in Crisis.
Green Lantern, Sinestro and his Sinestro Corps Lanterns have slaughtered many Green Lanterns throughout the years, especially during the Sinestro Corps War storyline.
The Joker's most infamous kill is the second Robin Jason Todd. This is such a sore spot for the Bat-Family (even after Jason's resurrection thanks to said Superboy-Prime) that when it seem that Killer Croc, let loose by the Joker during Joker's Last Laugh, had eaten the third Robin Tim Drake, Nightwing beat the Joker to death, Batman having to resuscitate him to make sure Dick didn't have that on his conscience, especially since Tim was alive.
For street-level characters in the DC universe, Lady Shiva dropping into your life is pretty much guaranteed to end in someone's death. Though she's rather unpredictable, and has even saved heroes on occasion, she's also been responsible for the (admittedly temporary) deaths of Batgirl, The Question, and Richard Dragon, among many less prominent characters. She's also come within seconds of killing Huntress and Connor Hawke (the second Green Arrow), and beaten the tar out of Catwoman, Nightwing, Robinnote who only survived by poisoning her ahead of time and even Batman himself.
While Lex Luthor's role as the Greater-Scope Villain in the Neil Gaiman-written Black Orchid included his goons who killing the original Black Orchid when they discover that she'd been spying on them.
Onomatopaeia is a downplayed example, targeting Badass Normal heroes like Green Arrow and Batman, and staying away from genuinely empowered foes.
While Solomon Grundy is often remembered for his usual third person speaking pattern he is a dangerous and unpredictable threat, in both terms of personality and the strength of his powers and has killed some heroes who have stayed dead through several universe reboots such as Skyman, who is better known by his Kid Hero name as the Star-Spangled Kid.
The supervillain known as the Mist his daughter Nash in Starman, the Mist kills the second Starman David Knight only after two days of being Starman, which leads to Jack Knight becoming the new Starman. Later on in Issue 38, Nash kills three members of Justice League Europe, they were: the Blue Devil, Amazing Man III, and the Crimson Fox (Constance D'Aramis).
Superboy-Prime, who during Infinite Crisis killed roughly a dozen named superheroes. Ironically, he also resurrected many superheroes when he punched the barrier of reality.
Wonder Woman (1942): After Paula von Guther had turned to using her Super-Intelligence for the good guys and become one of Wonder Woman's most stalwart allies she is killed by the Crime Chief while saving Diana from the evil cop.
Wonder Woman herself has actually been killed by a villain twice. The first instance was during the War of the Gods event in which Circe devolved Diana back into the clay from which she was made. The second was when she was killed by the demon lord Neron during the Hell To Pay event in Wonder Woman Vol 2.
In Volume 2, Hippolyta hosting The Contest in order to replace Diana as Wonder Woman after learning that Wonder Woman is fated to die soon leads to Artemis's brutal death as Wonder Woman at the White Magician's hands.
