My guess is that they were worried about slowing down because they knew other drivers behind likely would not, and they would get wiped out by a blinded driver going at high speed... ironically which is exactly what they did to the guy they hit.

You think it would be obvious to slow down when you can't see however there many videos of huge highway pile-ups due to fog where cars all go through fog way too fast and cause wreck after wreck... so it's clearly not that obvious.

I've been in a similar situation myself riding a motorcycle in heavy fog on a high speed road... you obviously have to slow down with the reduced visibility but the entire time will be a terrifying paranoia inducing nightmare of someone going full speed behind you just ploughing into you out of nowhere.

It's pretty awful tbh, all you can do is slow down as much as possible and hope anyone behind you is sensible enough to do the same. I definitely would not want to be in that position in a country with questionable driving standards.