Based on my observations I think the Hive are a race of necromancers who are able to control the bodies of dead humans, Cabal, Fallen, etc. They probably can't control the Vex because they aren't organic creatures. The Necromancers seem to have direct control of their minions, sort of like the Gravemind did with the Flood. But the Hive does not appear to be a parasite, so they probably use some form of dark space magic and a combination of technology to rip the souls out of the sentient organic life they capture/kill and use them as husks. This is similar to how in Myth Balor and the other fallen lords created an army of the dead to fight the Legion. Many of the Fallen Lords were also necromancers who used terrible and powerful magic. If the Hive necromancers do have direct control over their group of minions, killing the necromancer may kill or stop all the remaining minions because they have no guidance and are no longer bound to their master. Also we don't know how many converted the Hive can control at a given time. http://i.imgur.com/u25xGGJ.jpg If you look at the Hive in this picture, you can see "it" is instructing the dead on what to do. The Hive "necromancer" also strongly resembles as S'pht compiler from Marathon with it's long flowing robes. If you look in the trailer, it is evident that the Hive "modify" the victims they capture, sort of like the Reapers from Mass Effect and the Combine for Half life. They appear to remain mostly organic in nature but are infused with presumably a form of Hive technology and some dark form of space magic to animate their corpses. You can see here: http://www.bungie.net/pubassets/3857/10_1_Hive_5_shot.jpg That some of the victims of the Hive appear to have massive holes missing from the bodies and organs removed. It is not something most creatures (especially humans) could survive from. It was odd that they had that very large alien chained up after he appears to have been captured and transformed to serve the Hive. It could be possible sentient beings being converted by the Hive can resist their control. I doubt races like the Fallen could because their souls escape their bodies at death. Also Necromancy is one of the forbidden arts, like Geomancy.