The first step in Patanjali’s System of Ashtanga Yoga is ahimsa, which means non-harming. When you are Self confident the need to hurt, humiliate or kill another being is absent.
Only a person with low self-esteem would harm another to feel better about themselves. Self-esteem and self-confidence are the result of Yoga practice, and they have their highest manifestation in Samadhi.
Vegetarianism is the main tenet of ahimsa. You simply cannot intend to eat another being without harming them first.
A vegetarian diet uses up the least amount of natural resources and so causes the least amount of harm to the whole planet.
The more you practice ahimsa, the closer you come to the realization of your true nature: that which is peaceful and free of debilitating internal conflicts.
Many people have difficulty accepting the idea of a vegetarian lifestyle as intrinsic to the practice of yoga asana. To practice asana really means to practice perfecting your relationship to Mother Earth and all her manifestations.
The Sanskrit word “asana” means seat or connection to the earth. Earth means all things: animals, plants, minerals–all of existence. The Yoga Sutras States sthiram sukham asanam. (2.46) This means that your connection to the earth should be steady and joyful.
Yoga has been called the perfection of action. All actions originate as thoughts, so a perfect action must come from perfect thought. What is a perfect thought? A perfect thought is one that is free from selfish desires, anger and hate. We return to AHIMSA as the means to perfect action. See yourself in others, all others and go beyond seeing. BE yourself in others until there are no others, until there is only LOVE, only ONE. THE STATE OF ONENESS IS YOGA
-From The Textbook of Yoga Psychology -Shri Brahmananada Sarasvati
Himsa (harming) is classified in three divisions:
Physical, by body and instruments including war.
Vocal, by speaking against others, including psychological warfare.
Mental, by thinking against others.
Himsa is to act against, to speak against, or to think against oneself or others. To injure oneself or others in any form is himsa. It includes use of narcotics and all other substances injurious to body tissue. Abstention from all types of injury is called Ahimsa.
This term is used without adjective; it includes every type of injury because it is used in a broad sense.
One cannot injure others without first injuring oneself because injury is the result of psychological planning. Vocal injury is more serious than physical, and mental injury is most serious. By physical injury one can destroy only physical forms. By vocal injury one can destroy both physical and mental forms. By mental injury one can destroy even the form of spirit. Consequently one will go to a lower transmigration.