oh and by the way, Blizzard confirmed at Blizzcon that the only thing the dark voice changed about the overmind was making it focus entirely on the protoss. If the overmind had had his own way, then the protoss would have been ignnored due to a.) their status as failures that the xel naga ultimately rejected and b.) several key traits that would have made them incompatible with the zerg (their psychic link which is interconnecte yet still allows for a great deal of individuality that was at odds with the zerg's centralized hivemind)
One of the guys at another site said it best
(It was even stated at Blizzcon that the revelations about the Overmind aren't meant to turn it into some kind of good guy or make the zerg out to have been noble. From start to finish, the zerg always the same revenous Swarm driven by the same Overmind's underlying fanatical impetus to forcibly incorporate the strongest species they could find into the Swarm and exterminating those which offered no benefit. The change is that the single-minded, obsessive fixation upon the Protoss specifically was imposed upon the Overmind.
Which in a way it even makes a sinister sense from the beginning, since when one examines it the Protoss shouldn't have even qualified. Based solely upon its instinctive desire to improve the Swarm, one would have actually expected the Overmind to dismiss the Protoss due to its own creators having deemed them a fundamentally failed creation with an impure essence that fell short of the perfection they sought. Not to mention the very nature of psionically communal, yet still individualized Protoss evolution and the Khala is really pretty incompatible with the centralized hivemind of the Swarm.
Really, the greatest strengths of the Protoss are basically opposed by their very nature to that of the zerg. What use would the innately hierarchy-based hivemind of the Swarm have for a communal mind link in which every individual is an equal presence among the whole? The assimilation wouldn't have worked, as to consume them and even remain the zerg Swarm at all would mean not incorporating the very things that gave the Protoss their strength. There's no room for an Overmind in the Khala, because it literally is a linking of independent minds. The Protoss derive much of their evolutionary strength from their particular form of psionic interaction and energy, and that very thing wouldn't really even work within the Swarm.
That's the revelation: rather than simply spreading of its own accord, devouring and assimilating new races as it went to improve the zerg (which it was created to do), the Overmind instead found itself pointed at an impossible goal instigated for the purpose of making sure that the zerg clashed with the Protoss and allowed for other agencies to step in and take over the hybridization of the two. The very fact that the Swarm couldn't even infest Protoss as it could terrans shows that even had it remained alive on Aiur, the Overmind probably never could have successfully assimilated the Protoss as it seemed intent upon doing. Only the intervention of other forces has allowed for the merger of the two, and even then the results are hardly perfect (it's not like there's one ideal Hybrid with the best of both races; one's clearly more Protoss in nature and the other is more like the zerg, as if even the Dark Voice's minions can't really manage a truly balance merger of the two). When it comes down to it, the Xel'Naga themselves are probably the only ones who could effect a true merger of the two races, and so every other attempt is producing imperfectly blended Hybrids that skew one way or the other.
There's no reason to think the Overmind cared what otherwise might become of the terrans or Protoss. It only cared that the zerg were being steered toward slavery and eventual destruction, and wasn't some noble self-sacrificing martyr looking to save the universe. It created Kerrigan expressly to free the zerg and protect the Swarm from the fate it would suffer if left subject to the Fallen One's designs.) there's also the fact that tthe dark voice intended to wipe out the zerg when they were finished. The overmind didn't wish to save the universe. Just the zerg swarm. If he saved the universe in the process, then so be it.
