When I read the book, I assumed that he gave Marsh the Feruchemical atium spike, because he is about Ruin not just destruction. So maybe after he found his body, he simply intended to use Marsh as a one man killing machine. Not have everyone snuffed out by the Mists - instead he would make Marsh kill people one by one. That might take some time. Especially, since he would probably kill all of the people on the other continent. Still that plan makes a lot of sense from what we saw of Ruin's mindset.
