"Why are there no pictures?" you ask. Well, I can't find any good pictures that relate to the Cull at the moment.
The Cull was an event on Earth that turned two thirds of the population into Nether. Currently, there is no explanation as to why this happened, but there are a few theories that circulate around this event. There is even a religion based around this event that thinks the Nether are a test of humanity. It's called the Order of the Cull.
This theory explains that the apocalypse is the classic Juedo-Christian apocalypse. It would make a bit of sense, because the Nether seem to match the descriptions of demons, who would kill humans and rein hell on Earth.
This theory explains that the reason two thirds of the population turned into Nether is because a disease, possibly from beyond Earth, had spread into Earth, causing two thirds of the population to turn into Nether. There's no explanation why the entire population wasn't turned into Nether, and instead two thirds did.