Double barrel is a game of survival and rock hard one liners. The civilisation as we know it has come to an end. Now supernatural evils from another dimension (or other dimensions) have invaded the earth. Not as a full out attack, but through manipulating one person at a time. As a reaction. Angels and other esoteric beings try to strike back, making earth their battle field.
You are, at least to some extent, aware of this (in contrast to most other people who just blame the government or something even less reasonable – even when it is chewing on their nose). You work for neither side. You work for the Adder Association (effectively working under the label of Alcoholics Anonymous). You work for the snake, giving Adam and Eve the free will. Casting them out of the paradise of irresponsibility, inaction and determinism that Eden presented. The reason for this is that somewhere in your past the supernatural or esoteric powers destroyed your life. Redering you alone, aware and bitter as hell. In the AA you met people with similar experiences (and they sported free coffee). You choose humanity – and you’ll drag both the God and the Devil to the end of the world if that’s what it takes to restore the earth.