THEORY OF WHAT IT IS (SPOILERS)
I think it's PURGATORY........
The whole movie is an endless loop (DJ says "we're all on the same ENDLESS highway" and the movie ends where it begins) and each time the characters go through the loop they get the chance make the right choice and find redemption.
But.......the two bloody men have made the wrong choice too many times and so when the ground that opens up at the end, it reveals hell below and those winged floating monsters come from hell (literally through the bodies of the men's sins) to capture them and take them down to hell. The made the wrong choice too many times. The monsters oversee the other characters and make sure they make the right choice or the wrong choice and judge them, too.
The DJ Voice explains a lot of it if you listen closely......
"All you lost souls racing down that long road to redemption and all you sinners running from your past but heading straight into that pit of darkness up ahead" describes all of the characters. "They're going to try to stop you but you gotta say *beep* it and keep moving. Tonight might be the night you finally outrun those wicked demons once and for all." He's talking about the floating monsters from hell and it means the characters might make the right choice and get away. At the end he even says "You can always get it right next time." It's sort of like he teases each character since all they have to do is listen but none ever do. He's almost daring them throughout the movie to do the right thing and escape hell and find redemption!