Clayface is the best example in Batman lore. Everything he does: the morphing, the voice, that thing he does where he morphs his features back-to-front rather than turn around. the big kicker is the times he absorbed people inside him to kill them, which he once temporarily did to Wonder Woman.
And then there's the Clayface whose main power was to melt people he touched into bubbling puddles of protoplasmic muck, which is described as horrifically painful even though it's extremely fast.
In The Amazing Spider-Man Issue 236, a villain, The Tarantula, a couple issues after becoming an actual giant hairy tarantula creature during a failed experiment, takes his own life after jumping off a building. The shot of his corpse is very graphic for a comic in the 80's, with the giant spider's corpse over a large pool of blood.
The modern Black Adam is disturbingly fond of horrible deaths. In Infinite Crisis he pushes the Psycho Pirate's mask through his head and out the back. In 52 he kills Sobek by forcing his jaw open until his head is torn in half. In WWIII he literally tears a man's face off in order to kill him.
And this is nothing compared to what Sobek did to Osiris...
Deliberately and brutally done early in the Invincible series, when Omni-Man murders the rest of the Global Guardians. Horrendously. Lethal violence in Kirkman's series tends to deliberately be family-unfriendly: this was merely the first brutal example.
While Les Légendaires usually avoided making death scene look too gruesome with Bloodless Carnage in the first episodes, it was still good at making even those death scenes disturbing by having the deadbodies found with wide-opened, white eyes and a mouth similarly opened. In later episodes however, mainly after the Anathos Cycle, some truly gruesome death scenes were actually shown. Those include Anathos destroying a whole city with all its inhabitant with a island-sized canon, Kasino being killed by having his heart impaled with Tenebris' blade (though the heroes see it like if Jadina was impaling him on her hand, which made it even worst), while his female bodyguards were crushed to death under Shimy's uncontrolled elementary fusion. Jadina even got Impaled with Extreme Prejudice just before her reincarnation.
While trying to write a more realistic comic book Steve Gerber had his protagonist meet a young, chubby nerd who is beaten until unrecognizable in a school lavatory for tattling, taken to the hospital, treated, released, then, on his first day back, booted from behind while retrieving a pencil, falling, rupturing a recently-repaired organ, and finally, dying in the ambulance because it was stuck in traffic. And Omega The Unknown was a Comics Code-era book of the seventies.
Zoster's death in Paperinik New Adventures is pretty disturbing: he is disintegrated molecule by molecule on screen after being overwhelmed by Xadhoom's powers.
Neil Gaiman's The Sandman is famous for these. In just the first book, there are human remains coating the walls of a drug addict's home, a woman who stabs herself through the eyes, a man biting open another's jugular like a wolf, and a girl who uses a knife to draw pretty pictures into her arms.
Marvel's original 1970s run of Werewolf by Night had plenty of these moments. In the Giant-Size Werewolf #5 tale "The Plunder of Paingloss", at the climax of one of the darker and more surreal stories in this series (which, for its era, was saying something), furry Jack Russel gets his claws into Sardanus, a gigantic demigod who's revealed to be in reality a skinny dude in tighty-whities. The enraged werewolf "ripped him to shreds anyway" in a red-washed panel that featured the near-naked dude straddled by the werewolf, whose bloody claws were ripping the dude's belly open. The ribbons of gore and screaming look of horror on the face of Sardanus were in no way mitigated by the EC Comics device of single-hue coloring. The next panel featured a triumphant Paingloss and the werewolf standing over Sardanus' literally gutted body, as seen from a foreground view over the dead man's shredded abdomen. For 1975, this was about as terrifying as Code-approved comics could get. Someone at the editorial office was asleep at the switch when that issue went through!
In Wet Moon, Fall's father dies of (presumably) a stroke. At first we see him sitting on the dock fishing; when Fall calls out to him, though, blood dribbles out of his mouth and his eyes glaze over, more than a little disturbingly, especially considering that they had not shown up in the comic before and this was the end of Volume 1.
One of the few truly horrifying deaths in Superman comic books is the death of Mr Mxysptlk in Whatever Happened to the Man of Tomorrow?. Having been exposed as the true force behind the horrible events of the story, as well as the deaths of Krypto, Lex Luthor, Lana Lang, Jimmy Olsen, Pete Ross, Bizarro and the destruction of the Daily Planet, Mxysptlk stalks Superman through the ruins of the Fortress Of Solitude in his true form: a humanoid Energy Being. Realizing that he now has no choice but to break his vow not to kill, as there is no other way to contain someone as powerful as Mxysptlk, Superman threatens him with the Phantom Zone projector... and activates it, just as Mr Mxysptlk tries to escape to his home dimension. He dies screaming, torn apart between dimensions, right on panel...