Brandon Sanderson
Vin and Zane Watch Outside the Tent
Zane's cutting has an interesting evolution in the story. At first, I added him cutting himself simply because–well–it made a cool scene. Zane, cutting his own arm in front of his father, primarily to make the other man uncomfortable.
However, there's more behind this, and the cutting worked very well into the interaction of the different magic systems in the book. Particularly Hemalurgy. The pain makes the voice in Zane's head weaker. There will be more on this later.
I'll admit that I didn't originally intend to make the Zane-Kelsier connection with those scars on his arms. However, when writing this scene, I couldn't believe I'd missed seeing what a great opportunity I had. Vin already associates Zane with the Survivor; letting her see the scars on his arms gives her another powerful connection, particularly since she misinterprets where they came from.