Admittedly, he doesn't really have enough good villains to kill them off.

I think his Marvel counterpart, the Sub-Mariner, is in the same proverbial boat: Namor never really got his full measure of revenge against Llyra for her murdering Lady Dorma; and Aquaman never really gets revenge on Black Manta for murdering his infant son Arthur, Jr. Does he? From 1992, Aquaman #6, "The End of a Road" Written by Shaun McLaughlin, pencil by Ken Hooper, inks by Bob Dvorak, cover by Kevin Maguire and Bob Dvorak.

Mercy Reef wasn't exactly a tourist spot, even for underwater: as an infant, Aquaman had been left there to die, but was saved by the porpoise Porm. Years later, he would bury his son there. Now, Black Manta has just torpedoed it: he may be trying to pick a fight, but that seems like a bit much, even if he was the one that killed Arthur, Jr. Next, he was leading an army of the mermaid people of Tritonis to attack Poseidonis: they had their own bones to pick, and were totally willing to side with Manta. After hitting Aquaman with "a little something I stole from S.T.A.R. Labs," a surprisingly brutal electrifying headpiece; Manta then hits Poseidonis with another surface-world weapon: fear gas, adapted from the Scarecrow's! All of the Poseidonis's citizens turn on each other, with the exception of one F'ancha: he wasn't local, and also worried how he was going to explain being immune, but he manages to rally the people to help their erstwhile sovereign. (You could almost flip a coin, but as of then Aquaman wasn't the king.)

Iqula, the hero of Tritonis, returns to fight the troops that had sided with Black Manta; mentioning in passing he was probably going to have to overthrow his local monarchy for doing so. That leaves Aquaman free to go at Manta, who wants this fight over just as much as he does. We get a little flashback to a young Manta, that I'm not sure was ever referred to before or since: he was shanghaied as a child (from Baltimore!) and forced to be a cabin boy. It's strongly implied he was molested as well, or else forced to kill to defend himself; but he also had seen Aquaman as a boy, swimming with the porpoises and not helping him a goddamn bit. (He wouldn't have known to help, but you can see why Manta might hold a grudge.) Manta later mentions not being proud of killing Arthur, Jr; but that was his major victory and he had to act like it was the greatest thing he ever did, to own Aquaman. He also taunts Aquaman, that he not only "don't deserve my hatred...you don't deserve a wife and child."

For his part, Aquaman is virtually blind with fury not just over Arthur, Jr; but blaming losing Mera on Manta as well, feeling she left him because he didn't kill Manta. Manta, perhaps rightly, doesn't think that's why she left; distracting Aquaman long enough to stab him with a wicked knife from between the eyes of his helmet! He leaves Aquaman to die, as his Manta-sub is about to fall into a crevice; but Aquaman telepathically commands some "surgeon fish" to cut Manta's air hoses--his stupidly, stupidly exposed air hoses! Seriously, did Aquaman pull those out every time, and if not, why not? His suit filling, Manta sinks, with what could have been a good haiku to go out on, with a little editing: "...so beautiful. The seas consume my hatred. As they consume me..."

Porm comes to see why Aquaman hasn't gotten out of the sub, and he's a wreck, even notwithstanding the massive chest wound. But something worse may be waiting, as a crazed blond face says "Now you're mine!" at the end of the issue. This series was Aquaman's return to a monthly title, in time for his fiftieth anniversary; but wasn't as successful as his next, with Peter David, would be. (Manta would appear in that run, in Aquaman #29; the GCD notes the 'new' Manta debuted in Superboy #20!) On DC's wikia, this stretch of Manta's history isn't even mentioned, and this wasn't even the most hardcore death that entry mentions for him! Unless King Shark biting off his face didn't kill him, I suppose. Also, that entry mentions Manta was autistic, then later cured by Aquaman, but still a villain! Still, not unlike Sinestro or the Joker, Black Manta had long since crossed a line where I'd consider Aquaman killing him justified. Even if it wouldn't stick, and he had to keep doing it...