If you are involved in advertising, marketing, and branding at all, you begin to realize that advertising is really taking all the weaknesses of a product and making them strength of the product.
So, with Google, the engineers realize that it is impossible for a company with that much power and investment capital involved to avoid evilness, thus, they take the opposite of the company and make it the slogan, thus an evil company becomes a company with slogan of "do no evil."
This is kind of ridiculous. The "don't be evil" slogan originated in 2001 supposedly, when Google had $7 million in profit and very few employees. They were hardly a powerhouse, although things looked very promising at that point.
The founders of Google were visionaries who understood the humanity involved in any large organization. The very slogan was introduced to help them remember not to do bad things when they get big. Unfortunately, it hasn't worked out that well.
So, with Google, the engineers realize that it is impossible for a company with that much power and investment capital involved to avoid evilness, thus, they take the opposite of the company and make it the slogan, thus an evil company becomes a company with slogan of "do no evil."