It's an electric chair.
It sits behind a glass wall so that anyone can see it. Anyone can watch. There are theatre seats for them to watch from. This glass wall though, and the chair, are in a concrete bunker and the bunker is underground, sealed in the dark.The ritual of leading someone to the chair only happens at night, in the small hours, when the night is deepest.
The people are sleeping who could be watching. Maybe they wake just enough to realise the dark truth that they are willing to let someone die in that chair. Rolling over, they face an even darker truth. That there must be something in Humans so irredeemable that it can only be answered with death.