"Walk always in balance, and beauty."
Everything in our Universe has its opposite - This is never absolute though, only comparative. No one thing is completely Evil or completely Good. There are no black and whites. Each contains the seed of its opposite and they can be transformed from one form to the other depending on many factors. For example, cold can turn into hot; or the season changes throughout the year. Also, one cannot exist without the other. There's not absolute Evil or absolute Good.
In order to have a harmonious change, Balance must prevail for both. But this is rarely the case. Most of the times things are out of balance and they affect each other, too much of one can eventually weaken and/or consume the other. Conversely, one can also BECOME the other. For example, life changes to death. Hot changes to cold. Night changes to Day etc. One can change into the other, but not randomly. It is happening only when the time is right.