I don't own Dr. Horrible's Sing-a-Long Blog. Or Norse Mythology, obviously.
Penny is dead. This is a fact.
This is also a fact: Billy stays up late, hunched over a table covered with diagrams of the human body.
This is a possibility: there are also blueprints on the table for a machine which could do the impossible and bring a dead person back to life. He doesn't have much hope. His inventions never seem to work when he needs them to.
This is another possibility: instead of blueprints there are papers, covered with occult symbols. Contacting Hel, Queen of the Dead, is tricky, even for her brother, and he was always better at fiddling with technology than at casting spells. Not that he's any good at that either – if he was, Penny wouldn't be dead.
These are all facts: Billy has all the cash he could ever want. He is infamous; his crimes always make the front page. If he wants something changed in the city, all he has to do is make a threatening phone call to the mayor.
This is also a fact: Billy is unhappier than he has ever been in his life. He does not think he will ever be happy again if he cannot find a way to bring Penny back to life.
The first possibility: Billy cannot make his device work. Making the death ray only took a few days, but it is impossible to bring a dead person back to life.
The second possibility: Billy's spell works. The Queen of the Dead appears, looks out of place in his basement. He makes his request. It is denied. He begs his sister. She refuses. Hel does not make exceptions, and cannot do so, even for her brother.
Maybe one possibility is a fact. Maybe they both are. Any sufficiently advanced technology is indistinguishable from magic, after all.
Maybe it doesn't matter. Penny is dead. This is a fact.