Composition and lyrics
"Left Behind" was originally known as "Lust Disease": "An old song that we'd been playing around with before the first album," recalled frontman Corey Taylor. "We ripped it apart and rebuilt it. We wanted to have a melodic element in there somewhere, and 'Left Behind' had a darkness that meant it sat well with the other songs."
It was described by British magazine Kerrang! as "considerably more melodic than the rest of the album, providing a brief moment of respite from the outright nastiness elsewhere". Taylor recalls that the song was originally "much more melodic". Producer Ross Robinson accused the vocalist of "trying too hard" and encouraged him to make it "a little more guttural and put a lot more emphasis on it and just make it a smack in the mouth."
Taylor has explained that the lyrics inspired by a period during which he was homeless, living under a bridge in his hometown of Des Moines, Iowa. Speaking to VH1, he reminisced that "'Left Behind' is a song about a time of my life that was really hard – I was homeless ... it's funny how many people you meet when you're homeless ... you find out so much about them, and you rely on them so much, and then when things start happening for you, those faces disappear and they're gone," concluding that the song is "about leaving behind the lives that make up your past."