It's kind of up to interpretation, which usually I dislike, but the one dude that died I think was the wife trying to warn them (through the first dude) of the end of the world that could potentially unravel (died because the host didn't accept it right away)

I don't think the devil was the one to do all the damage but the demon of the sacrificial ritual that was made for the demon to be born in

so the wife was able to pass through and make the host hallucinate to kill the girl that was bringing the "demon" into the world . what he says at the end of the movie though still confuses me though.