The man from Gadara wasn't insane. He was demon possessed.

As for God working in our lives, just because he can is no guarantee that he will. And no guarantee that he will act quickly. We know from the book of James that God uses trials in our lives to make us who he wants us to be.

(James 1:2-4 NIV) Consider it pure joy, my brothers, whenever you face trials of many kinds, {3} because you know that the testing of your faith develops perseverance. {4} Perseverance must finish its work so that you may be mature and complete, not lacking anything.