All life is connected on a spiritual level; the higher the level of intelligence, the closer the bond that exists. The best way to picture this is to view the spirit of all life as an ocean. There may be many levels to the ocean with drastically different life forms in each level but all the levels makeup a single whole. What effects one level of the ocean can impact all levels.
This is the primary reason that you should always be respectful of all life, not just human life. One needs only to have a close relationship with a pet to know that they are self aware and intelligent. The only spiritual difference between a man and a dog, cat or any other animal is one of degree. As spirit is an outgrowth of mental evolution, the development of an animals spirit is related to its mental development and capacity. The spirit of lower animals may not be as advanced as mans - it does not mean that it is not there.
While indiscriminate killing is harmful to a person's soul, it does not mean that death to sustain life is wrong. Death is as much a part of the natural cycle of life as birth is. The lion is not going to lay down with the lamb just because of our belief that they both have a soul - the lion if given a chance is going to eat the lamb (or more appropriately the antelope), because that is the lion's purpose in life, in the ecology in which it normally exists, and that is what its instincts will drive it to do. Both the spirits of the lion and the lamb (or antelope) will grow during their lives, however long or short that may be.
Problems occur when man distorts the ecological balance in such a way that it is harmful to himself, the ecology and other species that inhibit it. The mass clean cutting of forest in order to develop grazing land for cattle, which ultimately denudes the land and drives out other species is a perfect (and unfortunately common in some areas) example of this. We should always strive to live in balance with nature and all of the other species, plant and animal, within it, recognizing that we are all interdependent upon each other for survival as well as spiritual growth.
Even though you may view yourself as a single distinct entity, you are really multifaceted. Of the three things that comprise you - your body, your mind and your spirit - your body and mind are what we usually consider to constitute a person; however your spirit is the true core of your being. It is the seed that you start with at the beginning of your life and the legacy that you leave it with.
It is a fallacy to consider your body, mind and spirit as distinct aspects of yourself. The melding together of all these aspects in the body of your person forms the living representation of you. When one of your aspects is sick or neglected, it is detrimental to your other aspects. You can no more develop your spirit by neglecting your body and mind than you can develop your mind by neglecting your body and spirit.
Likewise, when you want to walk your path through life with awareness and with the intent of growing your spirit, you must do so with the understanding that you cannot do so isolation. Only by working simultaneously to balance and develop all of your aspects and working to promote the growth and development of the spiritual energy that surrounds you, can you hope to achieve your maximum potential in this lifetime.