I kind of wish people wouldn't overthink this. They're the Borg Collective. They've been around for a long time. They're both scary and tragic. Why do we need a detailed explanation of their origins? It's much more fun to have room to speculate, in my opinion.
I'm more interested in their future than their past, anyway, and from what I've heard about Destiny, I don't like the direction they went with that ... I do admit I haven't read it for myself yet, though.