Vin Asks Ruin about Preservation
After this scene, perhaps you can see why I wanted so badly to spend some time with Vin and Ruin talking while she was imprisoned. I felt this was important enough that I was willing to stretch plausibility a tad to make it possible. (The spoiler in the chapter 54 annotation explains what I mean by that.)
The discussion of morality here is an important one, as I wanted Ruin and Preservation to represent forces, not moral poles. This is vital for various reasons in the underlying cosmology. If they represented poles, then that implied there could only be two like them. But, as they represent opposites, that leaves more room.
Preservation did betray Ruin. This brings us onto the shaky ground of the morality of lying to achieve a greater good. If as much were at stake as is here—the end of an entire world—then perhaps you'd betray someone too. (I love fantasy. Where else can you talk about the end of the world as a consequence of a betrayal and have it be literal?)
Ruin's consciousness—separate from his power—isn't a particularly nice being. But you can't much blame him, as there's very little that is left of the mind that once was. The force of Ruin has pretty well molded the mind to fit with the force's intent.