The phrase "women in refrigerators" comes from superhero comics, referring to a DC Comics storyline in the '90s where Green Lantern Kyle Rayner's civilian girlfriend Alex DeWitt was killed by one of his enemies, Major Force, and her body stuffed in Kyle's refrigerator. The trope of killing off a hero's girlfriend long predates Alex's death--it's often dated back to the death of Spider-Man's girlfriend Gwen Stacy at the hands of the Green Goblin in 1973, but the idea was old then: Artist and co-plotter John Romita Sr. get the idea of killing the hero's girlfriend from a comic series (Terry and the Pirates) that he had read when he was young. It wasn't even the first time Marvel Comics had pulled that stunt; I was a bit surprised when I read an Iron Man comic from a few years earlier where Tony Stark's girlfriend was killed during a superhero battle.
It was, of course, Gail Simone, later a professional comics writer in her own write who wrote the Women in Refrigerators list from which the phrase "refrigeration" is derived. Interestingly, Simone's list is not a list of bad things that happen to female characters to advance that story of male characters. It was a list of any bad thing that happened to a female character, without regard for how it affected the narrative. While most of the events on the list are female characters (either superheroines or civilians) who were murdered, raped, depowered (if originally superhuman), or in some other way personally victimized, there are some events that are the opposite of the current use of the word refrigeration--i.e., the death of a husband might be noted. It was only later that "refrigeration" specifically came to mean the death of a female character to advance to story of male character--presumably because all characters experiencing suffering, and there must be some way to distinguish between the suffering of a female character that fits into sexist tropes, and the suffering of a female character which avoids or even reverses them.
Penny's death in Dr. Horrible's Sing-Along Blog fits into the trope of a female character killed to advance the storyline of a male character, but it's written with an awareness of the trope, and the gender implications of the trope, that I find interesting--as well as an interesting subversion of the superhero-super-villain dynamic. I'm sure that having Horrible and Hammer singing “A man’s gotta do what a man’s gotta do” was written with deliberate irony. And who, ultimately, is responsible for Penny's death? Horrible, the sympathetic villain of the piece who claims to love her but stalks her and plans her life without consulting her, or Hammer, the asshole hero who is upfront about not caring for her? Penny's death is the “let’s be ambiguous about who is culpable for what” game again, as with Faith and Buffy’s killing of Allen Finch, and Faith being put into a coma (was it the stabbing, or the fall?). It seems like Captain Hammer had Dr. Horrible pinned and was about to kill him with the death ray when it backfired. Hammer was injured, Penny killed. So who is responsible? Dr. Horrible created the death ray. Captain Hammer was the one holding it and using it. But it’s not his fault it backfired. Or is it? He shouldn’t have just trusted Horrible’s equipment with civilians around, should he have? But then, Horrible shouldn’t have tested and used it with civilians, including the supposed love of his life, around. A twisty thing. But while both are responsible, I would place the greater blame with Horrible, who initiated the confrontation.
This may even have similarities with the death of Gwen Stacy, where some fans and creators argue that it was Spider-Man's botched rescue, not the Green Goblin throwing her off the bridge, that actually killed her. But in Gwen's death, the line between hero and villain is clear, and we are never meant to doubt that Peter loved her and had her best interests at heart (even as he hid from her both his own secret identity and that of their best friend's villainous father--information that might have helped her protect herself, had she known it).
Dr. Horrible is subverts all that. The hero is a cowardly bully who doesn’t give a damn about the girlfriend. The villain loves her—or does he? “Love your hair” is about the most specific compliment he ever has for her, and while Penny does indeed have lovely hair, that’s not real much to base true love on.
“Penny will cry but her tears will dry when I hand her the keys to a shiny new Australia”. Horrible isn’t interested in Penny, as a person. He doesn’t care that much if he hurts her. He can justify it.
One of the things that made Gwen Stacy Peter's ideal girlfriend was that she was a science major--a personality trait that was introduced and then never even mentioned again, despite Gwen surviving for many comics and real life years after. By the time of her death, Gwen's personality and simply disolved into that of the Ideal Girlfriend.
This is not the case with Penny. We don’t see Penny through Billy’s eyes. She’s not some ideal of femininity, she’s just an ordinary person trying to make the world a better place in her own, often useless, ways. It’s interesting that both Penny and Billy see the world as a reflection of their own circumstances—Billy thinks when he is unhappy, the world is crap. Penny, when she is happy, sees the world as good. Their desires to make the world better are, at some level, selfish, centered on the self. Neither of them can really see past themselves. And yet they try—which Captain Hammer, despite his reputation, notably does not.
Billy is the nice guy stalker who doesn’t get the girl at the end. On the contrary, he kills her, and she dies with praise of his rival on her lips (again a contrast to Gwen, who was unconscious during the confrontation between Spider-Man and the Green Goblin). What drives Billy to kill Captain Hammer is not that he is using Penny—Billy does not seem to give much of a damn about Penny’s feelings—but that Hammer is going to screw the woman that Billy wants to screw, and is doing it just to hurt Billy. Billy is right in telling Penny that Hammer’s an asshole, but he, Billy, is not much better.
That Dr. Horrible's-Sing Along Blog is less sexist in handling Penny's death in the 2000s than Marvel Comics was with Gwen Stacy's death in the 1970s is not surprising, but the story of Penny's death is not merely an updated version of the story of Gwen's death (that would be Alex DeWitt, who, having been created in the 90s, was given a strong personality and a chance to fight back before she was killed). It's a critcism of superpowered men who claim to care about the woman and civiliians whom they put in direct danger with their macho antics.
And yet—we still have a female character killed to advance a male character’s storyline. And the person we are expected to sympathize with most over her death is Billy: the man who killed her and made her the meaning of his life but never really knew her.