Ahimsa in action.

The Yoga Sutra is not presented in an attempt to control behavior based on moral imperatives. The sutras don’t imply that we are “bad” or “good” based upon our behavior, but rather that if we choose certain behavior we get certain results. If you steal, for example, not only will you harm others, but you will suffer as well.

The first yama is perhaps the most famous one: ahimsa, usually translated as “nonviolence.” This refers not only to physical violence, but also to the violence of words or thoughts. What we think about ourselves or others can be as powerful as any physical attempt to harm. To practice ahimsa is to be constantly vigilant, to observe ourselves in interaction with others and to notice our thoughts and intentions. Try practicing ahimsa by observing your thoughts when a smoker sits next to you. Your thoughts may be just as damaging to you as his cigarette is to him.

It is often said that if one can perfect the practice of ahimsa, one need learn no other practice of yoga, for all the other practices are subsumed in it. Whatever practices we do after the yamas must include ahimsa as well. Practicing breathing or postures without ahimsa, for example, negates the benefits these practices offer.

http://www.yogajournal.com/article/yoga-101/beginning-journey/