The Mists weren't ever trying to kill people. They were trying to Snap people, but the amount of trauma needed to get that to happen ended up killing people, mostly the very young/old/ill. The deaths were a standard bell distribution.
The Mists were basically ramping up and Snapping as many Allomancers as they could, in order to get ready to fight Ruin, if he were released by the holder of the Well. Once the Well is used, the Mists go back to a more neutral state.
Unless, of course, someone ends up releasing Ruin. Then the Mists kick into high gear and start Snapping anyone with a hint of potential, even those who normally didn't have enough to Snap (this last part is my theory).