Developing mass bioweapons to be used on the same planet they need to live on is nothing but pure hubris. Who is to say that the disease won't evolve to be even worse and the vaccine rendered worthless? I wouldn't think they'd necessarily come out ahead if they're forced to quarantine for a long time and rebuild humanity from nothing (assuming they don't die themselves).
But I don't have the kind of ego they do either, so maybe they think they'd be fine either way.