That is the question!
Blessings my Beloveds!
The question, I'm afraid, has no simple answer.
I remember when I first began practicing, the thought of engaging in negative magic never even crossed my mind. Wicca's basic tenet of "An It Harm None, Do As Ye Will" and the "Threefold Law" were as clear for me as the Ten Commandments are to any Christian. You reap what you sow.
Let me say, that there is absolutely nothing wrong with either of these concepts, as long as you understand that not everyone follows them. Greed, envy, jealousy, anger, in others, as well as ourselves, are all part of the human condition. For the most part, they are fleeting emotions that we deal with on a mundane level, without ill effects spilling over into our personal or magical lives.
If you are choosing to perform magic to effect change in your life, you are almost always emotionally vested in its outcome. Witchcraft is an empowering and at times, emotionally volatile practice.
There are going to be those situations that push the limits of your ethical patience. It's not a matter of if, but when this will happen. A friend betrays a trust, you are accused of something you didn't do or someone threatens your family. You are not just angry, you're pissed beyond all definition of the word. Your "normal" mundane reaction might include confronting your friend, defending your integrity or filing a restraining order to protect your family. You choose to react magically and you bring "livid pissed" to a whole new level!
There are no "controls" in magic. There are no absolute, predictable outcomes. Magic follows the path of least resistance. It is a matter of will, purpose and intention. If your intention is to cause harm, you better be prepared for the unexpected. This is Witchcraft you are sending forth into the World. A snarky comment on Facebook is not a good reason to use it! It's like opening the Pandora's Box of magical possibilities!
I am the last person you would ever hear lecturing on "Magical Ethics". To do so would prove me a raging hypocrite! Patience is not one of my greatest virtues. I have been known not only to bend the "Rule of Three" on a few occasions, but to break it in half, toss it into a cauldron and burn it...with little regret. The use of Witchcraft is a personal decision that can have serious and devastating implications if used recklessly.
Having said that, the Wiccan Rede, like the Bible is open to interpretation. It is not intended to be taken so literally as to allow its "grey areas" to preclude you from protecting yourself from harm. It is meant to be a guide to personal responsibility. If you feel threatened personally or psychically, it is your personal responsibility is to protect yourself and those that are important in your life. Don't worry that you may be performing "Black Magic". Magic is not defined by color, it's defined by the integrity of the practitioner.
Choose wisely, but never lay your power at the feet of another.
In Darkness, Light!
For your reading pleasure, I would suggest, "Protection and Reversal Magick (Beyond 101)" by Jason Miller.