What is badal? What is Malala's opinion of it?

From the text:

I thought of our Pashtunwali code, which governs how we Pashtuns live. One part of that code is badal—a tradition of revenge—where one insult must be answered by another, one death by another, and on and on it goes.

I had had my taste of exacting revenge. And it was bitter. I vowed then that I would never partake in badal.