Servitors are their depiction of the Great Machine( Traveler, [○] will be important later ), they didn't receive the gifts we did and they hate us for it. It isn't clear what attacked them during their collapse, we knew it as the darkness. Unlike the Fallen we had Rasputin that detected it and took action, the Hive wouldn't just kill a kell though. That kell was of House of Stone if I remember right, also there would've been some indication in the Worlds Grave by now. Now about it abandoning us, it was a possibility and Rasputin was very aware of it. When it was going through protocols it went over stopping [○] if it was to depart by any means necessary. We still don't really know it tried to and Rasputin crippled it, btw read Ghost Fragments: Rasputin 1-5 and all the lore is above.