A good slogan should say something. If the inverse of your slogan is invalid or the same as your slogan, then your (original) slogan is probably not saying anything.

If you aren't giving something else up, then you aren't saying anything. It's just platitudes.

"Do not be evil" is basically meaningless as a lot of evil is done with the intention of doing good. With that level of ambiguity, it is entirely down to individual interpretation.