Why is nature destructive? Why does disease exist? If nature is a model of how life could be lived, than why are some of the results of nature so harsh? I'd like to answer this from a shamanic viewpoint. It is my hope that this opinion will shed light on how nature could be viewed as both a model of living and also properly respected for its potentially harmful ways.
Nature has cycles of creation and destruction - life and death. The spirit of nature does strive to maintain this natural balance. We as human beings consciously or unconsciously ignore the natural balance with dire results to ourselves and our environment. That said, there are forces that introduce disharmony into the cycles. A person who is holding anger or sorrow disrupts their own "balance", and an illness occurs. The shamanist identifies the source of the disharmony, does what they can to correct it, and the person who is being treated then has an opportunity to become healed. This healed state is a state more in harmony with the native "harmonious" vibrations and cycles.
Nature also has ways of being self-correcting. A disharmonious influence, like the person holding anger or sorrow, is eliminated through an early illness. If mankind chooses to abuse the earth and not practice a concern for environmental issues, the earth will "correct" Us. There are American Indian myths of the world become so poisoned that mankind is killed off except for a few survivors. The earth will then slowly (or quickly) heal the disharmonies we've introduced, and the survivors of mankind will be left with the aftermath.
What is the spirit of disharmony? Is it a deity? While (typical) shamanism holds there beings/spirits that are driven to be destructive, they are not equivalent to a dualistic being of evil. They are equivalent to the Trickster, and in mythology the Trickster is affected by his deeds just as those who accept his gifts/influence. There are degrees of disharmony and there are degrees of correct for disharmony. All degrees of disharmony and harmony are parts of the spirit of nature. Abuse of nature can lead to destructive results, but these destructive results are not necessarily a sign of an opposing spirit.
Consider the forest fires that frequent land near housing developments. There can be incredible damage to both properly and lives. These seem very destructive and potentially "evil". In reality they are harmonious and part of nature's purging destructive cycle. The fires are especially bad in some areas because there has been years of fire prevention, and the undergrowth is very thick and hence very flammable. The intensity of the fire is a self-correcting part of the forest/fire cycle. Houses are destroyed. How many houses are in the wrong place? How many are on the sides of mountains where the natural wind drafts insure a fire that will move very quickly and with great intensity? How often does mankind upset the balance of nature and then bemoan the results? How often does mankind disregard the price that we will pay for our actions as we are the seem to seek to destroy the environment?
The nature based religions/spiritual systems embrace the concept of cycles (night/day, creation/destruction, hot/cold) instead of fixed opposing beings. Night does not oppose day. Night cycles into day and day cycles into night. Neither night nor day can "maintain balance" as they are both participants in the balance.
Hence the destructive face of the nature does not consciously target us. The balanced spiritual path recognizes the destructive side and the creative side, and those in connection with the breadth of the environment struggle to both help the Earth and thus help themselves survive. The human urge to survive makes people embrace the creation of the Earth (and hence the helping of themselves) while respecting, and sometimes avoiding, the destructive side.