The story "The Big Fat Kill" ends with just about every prostitute in town emptying guns into an alleyway to kill anyone who could possibly connect them to the death of a famous cop. It's one of the more... impressive images. Manute sums it up with his last words, delivered to the man who organized it: "McCarthy, you shit!"
Earlier in the story, Miho cuts off Jackie Boy's hand. And then plugs his gun so when he fires it, the slide goes into his own head. She is explicitly described as "toying" with him. And then, when Dwight instructs her to finish him already, Miho proceeds to "make a Pez dispenser out of him."
Miho actually does this a lot. She never kills someone when she can almost kill him and then destroy him. Family Matters has one particularly disturbing case where she repeatedly cuts a fat man until he's choking on his own blood, and then when McCarthy tells her to end it because they need to hurry up, she punts off his head.
Then there's John Hartigan finishing off Roark Jr. in That Yellow Bastard. As he puts it, he's eventually just pounding wet chunks of skull into the floorboards.
Marv pretty much embodies this trope. He basically made it his goal to kill every single person involved in the killing of his beloved Goldie, capped off with the twin slaughterings of the man who did the deed: he sawed one's arms and legs off, had a wolf partially eat him and for good measure sawed his head off.
In a possible Lampshade Hanging, HYDRA (from Nick Fury comics) calls its superweapon The Overkill Horn. Its function? Remotely activating every single nuclear weapon on the surface of the planet the whole world over.
Another example is when after Alexander Luthor lost a finger and got stripped of his powers. Joker comes out of nowhere and sprays him in the face with his acid spitting flower, fries his head with two electric buzzers repeatedly, then finishes him off by a point blank range shotgun blast to the head.
Joker just seems to be DC's go-to guy for being absolutely thorough in making sure a character is Killed Off for Real. Plus he was REALLY pissed that he didn't get in on the whole Infinite Crisis schtick.
Boy: Jesus, KM, how many times can you shoot two guys?
During the early years of the Cold War, atom bombs had a habit of showing up in fiction as the Finishing Move in situations where it'd seem like overkill, or even dangerously self-destructive, to use them. One example is the cover of the 1951 propaganda comic "Atomic War!◊", featuring two American bombers fighting a Soviet submarine, and the following quote from one of the pilots:
"His bomb missed! But even a near miss will get that red sub when I fire my atomic rockets!".
Punisher: Harry "Heck" Thornton. Hitman and all around Arkansas redneck. Heard a story about Harry that four state troopers managed to surround him once. He draws and kills three of them, the fourth one gets off a shot, Harry ducks it and shoots him dead. Dodged a bullet, so I use thirty.
Punisher: It was enough to kill most men, but with him it was just unfinished business. Barracuda was dead when you shot him to bits and shot the bits and burned them. Anything less just left nagging doubt.
Yet another example: in one story Frank Castle expounds on the capabilities of a minigun as an anti-vehicle weapon, then finishes by saying
Punisher: ....but if you use it on a man? You turn him into paint.
Brianna Diggers LOVES this trope so much that she made smart bombs with their own AI in them to "go boom" on "baddies". To date, due to her love of this trope, she has scared demons which once terrorized the planet and her bombs, hundreds of them, were able to knock out a giant Dynasty War Gigas. Don't even get started on how over-equipped she comes for a camping trip in an area with lots of target practi... er, wild monsters.
Her sister Gina is also fond of this trope at times, if less so.
Speaking of the Dynasty, they certainly follow this trope. The War Wind disables/destroys all of a planet's active defenses. Rio (the aforementioned crazy patriarch) takes this a step further with the 'Shield of the Patriarch,' which protects him from all attacks coming at him through normal time and space, while the 'Will of the Patriarch' is a faster-than-light magic laser that can change direction. Not to mention that in a flashback, what is likely a younger Rio is depicted as blowing up a planet/moon without external aid.
In the opening sequence of the first issue of Soft Desire, we meet a woman who is trying to steal a mysterious box. Because of this, a fight ensues with a guy who just won't die.
Alan Moore's Top 10: "Permission to use extreme force, sir?" "Kick her !@#$%ing ass, son."
Preacher: Pretty much Standard Operating Procedure for the Saint of Killers, whose standard response to just about everything is to get to work with two Angel-forged Walker Colts that never misfire, never miss, and never need to be reloaded.
War Machine's Mark IV armor can add more weapons to his armor (by using a complex system of powerful magnetic fields he can literally rip his opponents weapons apart and freely integrate them with himself). But even this form doesn't hold a candle to the armor he used against the Skrull fleet: an entire transforming orbital weapons platform/base. When you've got a cannon on your shoulder that can spit unstoppable plasma death into the face of an oncoming alien battle cruiser, then that, my friends, is overkill.
This appears to be at least part of the attitude behind the use of Sentinels in X-Men across media. In this case, it's fuelled by ignorance and fear. A mutant who looks innocent could be (and we the readers/viewers know in a few cases, definitely is) able to rewrite the very fabric of reality to suit their wishes. Of course, that may very well lead to Taunting Cthulhu or (in Harry Potter terms) tickling a sleeping dragon.
In one Lucky Luke album, Joe planned to do this to Luke (although he obviously didn't execute it): "One bullet for every day we had to spend behind bars!"
At the climax of the X-Men's "Dark Phoenix Saga'', when the Dark Phoenix suddenly reemerges, Empress Lilandra desperately invokes Plan Omega: destroy the entire solar system and pray they can kill Dark Phoenix in the process. At that point, Xavier has no choice but to order his X-Men to kill Jean themselves to preempt this measure.
After her creator/mother helped her escape and turned loose against the Facility, Laura Kinney aka X-23 could easily have killed Zander Rice in less than a second with her claws. However such a quick, clean death was way too good for him after the thirteen years of physical and emotional abuse he put her through. Cue Laura putting up the claws to beat him for ten minutes before leaving him to be blown up by the bombs she placed to destroy the building.
In issue 43 of the G.I. JoeMarvel series, Cobra is chasing down Storm Shadow's uncle, the Soft Master, who has infiltrated Springfield to find out who really killed his brother the Hard Master. On his way out of town in a stolen cop car, he barely gets past an oncoming train, only to crash into a drunk driver who had passengers that just happened to have Cobra connectionsnote Crimson Guardsman Professor Appel's daughter Candy, and Cobra Commander's son Billy. The drunk driver's car catches fire, and everyone inside is unconscious; when the Soft Master tries to rescue the others, Scrap-Iron, who has climbed a phone pole on the other side of the tracks, fires an anti-tank missile at him. The Soft Master, realizing his life is already forfeit, jumps into the missile to keep the blast from killing the others. Unfortunately, Scrap-Iron still had a missile left, and with Firefly's permission, he uses it to blow up the car.