This goes back thousands of years in the tantric tradition; that you should always strive to avoid violence and doing harm to others and yourself. This is of course a very sensible rule, but a very interesting one from a spiritual perspective. Tantra tells us that we should avoid violence in both our thoughts, words and actions. Because everything is connected, even our thoughts of doing harm to others, or negative thoughts about ourselves, are an act of violence. When we do so, we emit a violent field of thoughts. We emit violent energy, and even if we do not act upon it or speak it out loud, we attract violent energy. As long as we have the violence inside of us, we also find violence surrounding us some way or another. We can only move on in our spiritual development when we do away with this. We get rid of it by beginning to to purge ourselves of it, and becoming aware of how much we can hurt ourselves when we think negative thoughts about ourselves and others. At the same time, on a subtle level, we are damaging that other person, because we are projecting negative energy towards him/her. From a spiritual perspective this will always be something that holds us back. In tantra we are working with the awakening and release of more energy and to allow the energy to flow more freely. Therefore it is natural that we are interested in clearing out all negative trends, both consciously and unconsciously because when we awaken more energy, we pass that energy onto what is already there, this cannot be avoided.