Quite simply though, Cube wants to die. It's an odd position he's in - there's no real way for Cube to win (Unless, by some random freak chance he's the only one left standing - which is very unlikely.) Cube's whole goal is to cause mayhem and mass destruction. So, if the BA backstabs Cube on Day 1, that's fine... so long as Cube's death kills many, many people.
And since the BA has the same "goal" as Cube (mayhem and mass destruction), when things are said and done, the two of them will simply be working together to get the entire town killed.