"An it harm none, do as you will" is basically an unfinished thought.  It only covers half the ground that needs to be covered before it can be applied in the real world.  If we were to take the other half of the statement, the part that is left out, beginning this way:  "An it cause harm..." then what?  This is a classic "there-are-two-kinds-of-things-in-this-world" statement.  There are those actions which do not cause harm and there are those actions which do cause harm.  The rede has only given advice on the first; if it harms none then do what you want.  What then, if it does cause harm?  Where's the advice in the Craft for those actions, which are much more common than the other type.

Well, since we have reversed the first part of the rede to get more actions available for us to commit, in other words those actions that cause harm, let us do the same thing with second half of the statement.  We would then get this:  "An it cause harm, do not do as you will."  In other words we are left with actions by which we are compelled. We cannot do what we "will," or want, in all situations. Anyone with the least little bit of experience in life knows this to be true. The rede to cover all possibilities should then be phrased like this:  "An it harm none, do as you will. An it cause harm, do as you must."  This would mean that harm is only justified when the situation compels you to commit it. For example, we may not want to harm anyone, however, we may be force to defend ourselves or our loved ones when someone else decides to do us harm. We cannot control their actions and it is unnatural to just allow ourselves to be injured, so we are "compelled" to cause "harm" in self-defense; we therefore are justified in doing "as we must."

"An it harm none, do as you will. An it cause harm, do as you must."