Having a coherent system of ethics is a defining characteristic of a witch. Please notice that I am not defining what is included in that system, that too is a defining characteristic of witchcraft.

I see ethics as the small, inner, ring of decisions we make as we live. The next larger ring is that of morals (the externally imposed values of society), with the largest ring enclosing them both indicating (for lack of a better word) Divine Law. This larger ring could be called karma, but is more easily described as the things we do not do because they would offend God/dess.

Faith and reason are like two wings on which the human spirit rises to the contemplation of truth; and God has placed in the human heart a desire to know the truth—in a word, to know himself—so that, by knowing and loving God, men and women may also come to the fullness of truth about themselves (cf. Ex 33:18; Ps 27:8-9; 63:2-3; Jn 14:8; 1 Jn 3:2).
~John Paul II

Replace God with God/dess and we have an apt description of the search we each undergo to discover our ethics, and how to live by them. In every part of the world, in every culture we humans have questioned why evil exists in the world, and in searching for those answers we become even more our Selves, closer to our true natures, and therefore closer to the Divine within.

This page links to a series of articles and writings about the various attempts to define the ethics of various witches.   In no particular order, this page offers: 
•    Four variations of the Rede.  Although (to my eye) they differ only in the smallest details, I have put them together for comparative purposes. 
•    Several collections of  Goals and Rules that I've collected over the years.
•    Karl Lembke's 'Rede of the Wiccae' reminds us that we belong to a religion of responsibility, as does Judy Harrow's "Only If None Be Harmed: Getting Specific about Magical Ethics." 
•    I offer two somewhat scholarly Discourses on the Rede for comparative purposes (One and Two) as well as Judy Harrow's 'Exegesis of the Rede', a wonderful look at the Rede using investigation techniques from other religions.
•    For entirely different reasons, so is J. Crowley's 'My Craft Ethics'. 
•    Michael Walden's 'Perfect Love...' should get you to stop and think about our password. 
•    I have a couple of pages of Laws for your perusal (The Laws, and Galadriel’s), as well as JaguarMoon’s.  Aside from my own coven’s (of course) I do not recommend any of them in particular, but I offer them to you so you can see what other covens and groups feel were necessary to codify. 
•    I also have an essay on the 'Beliefs & Customs of Wicca,' another on 'Basic Principles,'  and the American Council of Witches’ ‘Principles of Wiccan Belief’
•    Finally, I have included a few other odds 'n' ends: JaguarMoon's Expectations, Integrity, from a Reiki Master, and Brandy Williams’ piece on Open Circle Ethics.

Ethical teaching is one of the first lessons in becoming a Witch.  Below is my own way of walking within the world.

Maat's System of Ethics 

1.  An it harm none, do what you will.
 2.  Never speak falsehood.
 3.  Bear in mind that the act of withholding the truth is always potentially a lie, and a significant moral decision is required each and every time you do so.
 4.  The decision to withhold truth should never be based on personal needs.
 5.  The decision to withhold truth must always be based upon the needs of the person from whom the truth is being withheld.
 6.  The assessment of another's needs is an act of responsibility that is so complex, it can only be executed wisely when one operates with genuine love for the other.
 7.  This assessment must be undertaken with the knowledge that we tend to underestimate the capacity of another's strength.
 8.  Trust is earned, not given.
 9.  Treat others with the dignity and respect with which you desire to be treated in turn.
 10. Love yourself before all others.
 11. Speak thoughtfully, but openly, and do not worry about what others think – it is your life to live, not theirs.
 12. Give back more than you take.
 13. Walk upon the earth lightly, honor her as your first ancestor.
 14. Value yourself and your services fairly when compensation is involved.
 15. When given work to do, do it the best you know how.