All organisms demonstrate a tendency to avoid harm. Even amoeba will avoid aversive stimuli. This is one of the basic premises of behavioral psychology's operant conditioning. Behaviors that yield pleasing results tend to be repeated. Behaviors that yield aversive results tend to not be repeated. Amoeba have no need for morality, only self-preservation.
If we were to stop right there, we would have an argument for hedonism, indicating no need for morality. But, we are not amoeba. Humans are social animals requiring the assistance of other humans in order to survive in the natural environment. For humans, self-preservation is interdependent with preservation of "the tribe."
Other social animals like wolves, lions, and buffalo will predictably behave in ways that promote the health and safety of the group over the health and safety of the individual. These animals engage in what might be considered benevolent behaviors even without the benefit of higher cognitive functioning.
To my knowledge, humans are the only species capable of true empathy. Empathy does not mean sympathy. Many species demonstrate sympathy. Empathy requires the complex ability to cognitively attempt to see through the eyes of another. With huge effort, it is possible to put our collective ego aside and on some level understand the world from another person's perspective.
Research on feral children has shown that empathy is a learned behavior. Empathy is an extremely difficult cognitive skill that few humans try master. If humans developed and regularly employed this skill, conflict with each other and the destruction of other species could be virtually eliminated.
My theory is that empathy is the highest human good, as it is an extension (actually a giant leap) of the natural tendency for social animals to engage in behaviors that benefit the survival of the group. To employ this litmus in making moral decisions, one must not only, "do unto others as you would have them do unto you," but also consider how others would wish to be treated from "their own" perspectives and not just your own.