Expanding what TaCTiCaLxReFLeX
Red has basically been in the mine since he was 14 I think and has been trapped in there since, so about 20 years I think. As you get near the end, you find Red inside an incinerator as he wants to kill himself. He has been suffering from cabin fever which is when you are in a small place for so long you go paranoid and you sorta go insane. (look up Wiki for more details.)
So after you kill him, you get a key and find more about him, like a few notes, some books, a noose (suggesting he tired to kill himself) and you escape to "The Shelter." There you walk down some stairs, enter this hallway and see this black figure at the end (The Infected from Black Plague) as you walk to it, the lights go out and you eventually get knocked out and the game ends...leading onto Black Plague.
Just started it and it's very scary. If you have any more questions, let us know. Also check out my channel bellow and look at the last 2 or 3 videos of overture for the ending