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.
Going by that logic, just a meter deep is a crushing death. And right where you are you could smash your head against something... which is at least also something that happens all the time, no need to look at theoretical far-away things when the boring truth is far worse.
Just a few days ago, someone I know had to be hospitalized because of somehow slipping and smashing badly into their desk while doing normal, boring office work. From how it was described to me, I cannot help thinking sex was somehow involved, like receiving or giving oral sex from under the table maybe.
EDIT: Typing this, I was reminded of a guy who got smacked in the back of his head. Got X-rayed, everything looked normal. They kept him in the hospital for observation, though. X-ray again after an hour, a huge hematoma had evolved under his skin, pressing on the upper spine. They immediately drained it. Otherwise, it would have killed him very quickly.
You don't have to get to the karman line to asphyxiate, the karman line is only relevant if you are trying to move faster to generate lift. The Death Zone starts at only 8 km altitude.
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.