No, the Mists were not a danger in the sense that the people of Scadrial thought that they were. From the Hero of Ages Epigraphs: Chapter 81 (Emphasis mine)
Snapping has always been the dark side of Allomancy. A person's genetic endowment may make them a potential Allomancer, but in order for the power to manifest, the body must be put through extraordinary trauma. Though Elend spoke of how terrible his beating was, during our day, unlocking Allomancy in a person was easier than it had once been, for we had the infusion of Preservation's power into the human bloodlines via the nuggets granted to nobility by the Lord Ruler.
When Preservation set up the mists, he was afraid of Ruin escaping his prison. In those early days, before the Ascension, the mists began to Snap people as they did during our time—but this action of the mists was one of the only ways to awaken Allomancy in a person, for the genetic attributes were buried too deeply to be brought out by a simple beating. The mists of that day created Mistings only, of course—there were no Mistborn until the Lord Ruler made use of the nuggets.
The people misinterpreted the mists' intent, as the process of Snapping Allomancers caused some—particularly the young and the old—to die. This hadn't been Preservation's desire, but he'd given up most of his consciousness to form Ruin's prison, and the mists had to be left to work as best they could without specific direction.
Ruin, subtle as ever, knew that he couldn't stop the mists from doing their work. However, he could do the unexpected and encourage them. And so, he helped make them stronger. That brought death to the plants of the world, and created the threat that became known as the Deepness.
So yes, Ruin manipulated the Mists to serve his own purposes, just as he manipulated the prophecies to create the legend of the terrible Deepness. This in turn furthered his goal, which was to have someone go to the Well, and release him.