On Nissa's XelNaga vs Another Race:
I see this in our own world. Some secret few want to control everything, even their own kind; some want to have all the power, control or not, they're on top; Some just want to experience nature, and take some of nature home with them in their buckets; some just want to watch nature; some want the cool new things, and don't mind what damage they do to nature, animals; some want to manipulate DNA to grow ears on them, organs for their use.
I'll add Mr. Scott's Engineer's into the equation, and our own debates about trans-humanism and genetic engineering.
The perfect weapon is created, the Zerg, at least from the view of one faction among the Xel'Naga. To others this is proof eternal life can be achieved: if one can transfer one's consciousness in the network of consciousness through purity of essence, and live in perfect form, one has achieved eternal life.
Others see this as an abomination; others see this as meddling with things that shouldn't be meddled with, etc.
The Xel'Naga were viewed as gods, but they are not. They're ancient, but nothing new is under the galactic black sun. Death is inevitable, life is meaningless, maybe if we defeat death, we find meaning. But it's all vanity. Thus, starcraft.
Having a Xel'Naga with differing thoughts would make them more interesting. Why they wanted purity of essence, at least in their varying definitions of it, becomes a source of contention. Goes for purity of form as well.
Consider the Temple on Shakuras. It could have been made to wipe out their own creations if ever they had to. Either by necessity, or if something goes wrong. Or it could be some other faction's bomb waiting until the creation of the Xel'Naga ever succeeds -- a way out for someone else or themselves.
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What do you think? That's all I can add for now. Haven't got much sleep. I'll think more.