Answer:
I am a box that holds keys without locks, yet they can unlock your soul.
Used left or right, I get to travel over cobblestone or gravel. Used up, I vie for sweet success, used down, I cause men great duress.
What word has kst in the middle, in the beginning, and at the end?
Both punishment and sweet reward, It will befall both fool and lord. It is a spiritual scoreboard, Your every deed it does record.
What goes through a door but never goes in. And never comes out?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?