One theological foundation for nonviolence to all living creatures, including adopting a vegetarian lifestyle, is to be found in the idea of the Divine Breath (Ruach HaQadosh = Breath of the Holy One).
Breath maintains life and so is essential, is at the very essence of any breathing creature. By analogy, the Divine Breath is the inherent Divine Life Force, is at the very Essence of G-d. The breath of any breathing creature also can be projected beyond that creature, and can form the substance of sound and word coming from that creature. By analogy, the Divine Breath also can be projected and form the substance of sound and word from G-d. So, while remaining One with G-d at the level of the Divine Essence, the Divine Breath also constitutes the substance of projected creation from G-d. Thus, through the Divine Breath, all is continuous, from the Divine Essence to every creature, even in any and every difference.
This continuity means that one should see the Divine in all and see oneself in all. Therefore, as one loves and reveres G-d, so too one should love and revere all, and as one would wish oneself to be treated with kindness and compassion, so one should treat all with kindness and compassion.
