Lit How about a Light side type of villain?

Which is precisely how GL states it in The Making of Episode I. For metaphysics buffs, I tend to use "Transcendent" and "Immanent" when talking respectively about the Unifying Force and the Living Force.

We also now have the most concrete evidence ever that the dark and light sides of the Force are interdependent polarities in the Living Force from the Mortis episodes. What with it's giant Yin/Yang diagram. Which is what the Dark Empire Sourcebook had always said, and RPG books before them.

I've speculated that it could be not that way, but it is and that's sort of the end of that.

So I think we can safely say that on this issue the JATM is wrong and the other sources are right.

Sorry, Wookieepedia.

I wonder what would have happened if Son died and Daughter lived...

^They all died. That was sort of the point. If one goes out of whack the whole order of the universe does.

But what would an out-of-control Light Side look like?

Who can say? From the qualities associated with the light side of the Force, I'd say it's not really in the nature of the light side to become out of control. It's theoretically possible, but highly unlikely.

I guess some people within the Star Cabal considered themselves Light Side.

Depends, really. They'd not do anything stereotypically EEEEEVVIIIIIILLL like a jabbering nutjob, but -- evil drugs company aside -- Miracle Day was a good example from Torchwood of how "healing people" isn't always in their best interest.

Certainly, you could argue "saving them" was a selfish, dark side act, and, yeah, that's definitely true using Cade's evil dark side power, but if Cade went around being a saint using his light side power to save everyone he could, there'd come a point where that wasn't actually good for society, and only caused more pain in the big picture.

That's true and that's why Jedi who intend to remain true to the light have to be careful about how much Force power they use, because eventually they stop applying the necesary judgment to handle it properly and it becomes an act of darkness and they slip over without realizing it. In a way this is what happened to Jacen, who became so convinced that he could fix the universe that he changed from being supportive to being domineering.

The trick I think, has to do with the nature of simple vs. complex systems. The scenario of total dark triumph is a simple system - ultimate entropic triumph, the heat death of the universe or perhaps its total implosion, but regardless completely undifferentiated nothingness.

The scenario of total light side triumph is a complex system: some sort of perfectly peaceful, perfectly creative, perfectly just, perfectly loving galaxy. I'm not even sure what that would be, never mind how it could be achieved, but it is clearly not simplifistic. It's not just pumping endless quantities of endless 'good' energy into the system.

As a result Jedi, and other light side oriented Force-users, have to be very careful about how they use their powers, because pumping too much energy into any complex system, regardless of its direction, can be destabilizing. This is, I think, the basis behind Obi-Wan's 'Only a Sith believes in absolutes!' line in ROTS.