When everything goes to hell after the opening of the homeless shelter, Billy knows what to do. He knows that every villain needs a tragic backstory and he couldn't be where he is without a heartbreaking loss. But Billy was never a person who knew when to give up and walk away. He intends to cheat.
The freeze ray wasn't working the moment Penny was dying but at least he could use it to preserve her body. Penny would be waiting for him indefinitely. Billy reasons that his status of a mad scientist comes with consequences. He knows who is the archetype and he understands that the popular correlation between science and evil didn't come from the works of philosophers questioning modernity after second world war – it started at least a century earlier with Mary Shelley. The scientists make perfect villains because they disobey God's will. They ignore the laws of men and every sacred scripture. They know how to cheat, and that's what makes them special.
He doesn't name the cat he kidnaps from the animal shelter. He's learnt his lesson years ago. He considers testing his theories on an already dead one but he needs to be sure. He must know how the animal acted before its death, how it moved, how much it used to sleep and what it liked.
This is not pleasant. He would prefer to do this quickly, to never know the cat and to test his invention the moment he brings the animal to the laboratory. But there would be no point. He must know. When the cat purrs on his lap he very carefully doesn't think of it as "Penny".
Through all these months he's able to come up with only one way to resurrect the dead and he realizes with panic that if it doesn't work, there are no alterations and no new equations he can do to make it right. Even if he spends every waking hour of the rest of his life running the calculations, he won't come up with anything. There's nothing else to come up with. The test will be final and perhaps that's why he keeps postponing the experiment. He double-checks every symbol and every shred of thought. The cat sleeps in impossible places and positions, rubs against his legs, and occasionally chases a skein of wool.
It doesn't hurt, Billy makes sure of that. The animal falls asleep and, in contrast with Billy, doesn't even feel any nausea.
Billy feared it and the suspicion paralyzed him but when the cat doesn't wake up he still feels cheated somehow. He buries the animal in the public park just before the dawn; there's a small ceremony of sorts, and he's the only one attending. He's not sure if he's holding it for the unnamed cat or for Penny.
He gets another cat.
And then another. And there are many to come, for Billy is no longer a good man.