You discount the people that want to watch the world burn. The tree of...fed with blood...blah blah blah. "Burn it down, start over" is often touted as the fastest/best approach for wholesale changes when the friction to making change is too great.

Economically, China would probably be the biggest beneficiary to a US civil war, especially one that ended with 2 Americas with neither the strength of the former union. Russia would just love to see the chaos and reap whatever gains they could get as well.