Answer:
A dragons tooth in a mortals hand, I kill, I maim, I divide the land.
My second is performed by my first, and it is thought a thief by the marks of my whole might be caught.
What goes through the door without pinching itself? What sits on the stove without burning itself? What sits on the table and is not ashamed?
Look into my face and I'm everybody. Scratch my back and I'm nobody.
Gets rid of bad ones, short and tall. Tightens when used, one size fits all.
You use it between your head and your toes, the more it works the thinner it grows.