The way I look at it, Ruin and Preservation were stuck on an empty mud-ball.
Preservation wanted to make life, and Ruin was like, "Sure, but I get to break it later."
Preservation agreed, but then added a spark of a shard to pre-sentient humans, causing them to get smart. (See my signature for Shardic Sentience)
Ruin got mad, but then Preservation sealed him away.
Cue Mistborn series.
on the OP: I always thought of Ruin as well, chaos (D: Chaos is Ruin!), and Preservation as order.
Chaos provides the primary creative force, since chaos encompasses all possible outcomes. However, being chaos, it has no structure. This is where order comes in. Order provides the structure for the creative force, but order alone is like a blank page with no pen. With no reason for change nothing will change regardless of the positives/negatives of the current state.