Statistically if everyone is 5% evil, the chances of someone being evil to you in the course of the day is pretty high. That sounds like the makings for a downward spiral and "don't be evil at all" is much safer for society.

Obviously there will be people who choose to be mostly evil regardless of what everyone else is doing, but society trying not to be evil in general is still the best case scenario.

I take a penny but I never leave one.