As a standalone story, The Killing Joke is textbook fridging. The Joker shows up at Barbara Gordon's apartment, physically and sexually assaults her, and as a direct result Batman is finally pushed over the edge (or so it seems...) to give readers what is considered to be one of his greatest stories. Alan Moore has since disowned it, and Barbara's eventual rebirth from a battered Batgirl to the organizational heart of the Bat-family as Oracle enabled her to deal with her own trauma and remain an indispensable player in the Batman mythos.
Similarly, Earth 2's Lois Lane's death appeared to have been crafted solely to amp her husband up a few gears towards his noble self-sacrifice, but wound up giving her new robotic life as Red Tornado. (In a possibly self-aware twist, her back-up body even emerged from a cryogenic chamber, having been literally fridged.) And yet, Simone's original question still hangs in the air, unanswered, as to "why they were thrown in the wood chipper in the first place."
It's important to also note that fridging doesn't instantly make a story bad. Thanks to Gerry Conway's nuanced writing, Gwen Stacy's murder in The Night I Let Gwen Stacy Die, one of the earliest examples of the trope, has a more complex emotional effect on Peter Parker's character than just swearing an oath of vengeance, and has had far-reaching effects in subsequent Spider-Man stories. Gamora's murder in Avengers: Infinity War successfully builds Thanos into the mournful maniac that the Russo brothers promised us he would be.
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Fridging can be an ugly blemish on fan-favorite stories or make lesser ones look even worse. But in both cases, it boils down to female characters being disproportionately cast as disposable pieces of meat who can only contribute to the plot when they're stuffed inside a refrigerator. Even when you're aware that it's an issue, it's difficult to make work. Deadpool 2 openly called out its own "lazy writing" rather than attempt to make Vanessa's fridging amount to more than just a James Bond parody and a sexy, ghostly rendezvous. Unfortunately, despite the film's post-credits retcon, the image was already done, and no amount of jokes in the opening credits can make it go away.