Orcs and Goblins
Goblins come out of rocks. They've been there since nearly the beginning of time, when the world was fire and Chaos, before Order got to grips with things. The world cooled, and the seas of fire stood still and turned to stone with much of the Chaos of the world trapped inside.
Not all stones contain Chaos. But some do.
A Chaos stone looks just like any other stone. But when it is broken open, a strange thing happens: it grows a Goblin. Goblins can be made of anything, really, but most Chaos - real, pure Chaos, not the diluted kind that drifts around the world today - has faded, so most Goblins grow from rock.
People are a mixture of good and bad. They are a hodgepodge of Order and Chaos, and thus they are capable of exercising both aspects of their nature.
Goblins are bad. A goblin cannot be good, kind, or orderly, as that is not within them. They are not people. They are simply a manifestation of Chaos, and they act as such, causing wanton destruction and clashing with Order wherever it is found.
A goblin can be destroyed, but that brings problems of its own. The body of a goblin is just a vessel for the Chaos, and destroying it causes the Chaos to spread. It becomes dilute and will never form a body again, but it eats away at Order like an acid. Where a goblin is destroyed things become worse. Buildings crumble, books rot, and fields become blighted. But worst of all, people become orcs.
An orc is someone whose Order has been burned away. It is a sorry thing to happen to a person, as it cannot be undone. They simply become bad. It is best to lure a goblin into the wilderness first before destroying it.
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Alas, the stonemason. No headsman's axe is so deadly as your chisel, though you know it not.