An evil, or a very, very powerful ally? If the Didact returns, maybe the Librarian (through her gene/space magic) directs the Chief to one of the most powerful weapons the Forerunners ever devised besides Halo in order to stop him. Overkill?

I imagine that whatever former Covenant forces who were on the fence about joining Jul's army before began flocking to his doors when they learned a living, breathing god had appeared and blessed his troops. With whatever insurrection trouble Humanity has been facing, not to mention rebuilding from the war still, a near fully-reformed Covenant being lead by Promethean forces could squash us for good. Not to mention if the Flood begins to creep its way back into our lives, which I believe it will. A revival of Mendicant Bias could be just what humanity needs.