I've always thought that the "Don't be Evil" mantra/motto/whatever is akin to people in denial telling themselves that they are "good people".
If you need a rule to tell you not to be evil, you're definitely not good people.
I don't agree with that. It's a statement that sets the company's policy. While obvious that people should not be "evil", a company motto setting it explicitly to be good is a welcome thing. Of course, Google is no longer that company, and they removed that policy statement after it became a mocking tool for critics.