I would argue that "instantly killed in a horrifying way" is oxymoronic. I don't care that I'll be torn to pieces by rabid centipedes while being boiled alive in a vat of acid, as long as it's an instantaneous death. Horrifying, maybe, to observers, but any instantaneous death makes the means irrelevant to the killed.
First, I don't think this was instantaneous. Depends on how high the voltage was.
Regardless of that, I wanted to point out that I once had a third-degree burn next to my ankle made by the escape pipe from a motorbike. It barely hurt at all, and it smelled like steaks. Thing is, the nerve terminals burn, and so you don't feel any pain. But it was ugly, following days I had to keep it clean... And the mark lasted a few years.
No matter where you are on the planet, you’re never much more than 100 km away from the Kármán line. Death is (generally speaking) always just overhead.
For your body to stay alive, a lot of things have to go right for the “engines” to keep running.
On the other hand, just a few things going wrong is all it takes to take down the overly complex Rube Goldberg machine that is your body; all being inevitable due to how entropy and randomness work in our universe.
The voltage difference between the two terminals would be minimal so no current flow, the back plate should be non-conductive. The other reason is there are no scorch marks or other signs of burning.
Now if the mouse had gone between the green and yellow (where the terminal separator is); then we would see a big voltage difference.