Good is restricted from doing certain things, and encouraged to do others.
Evil is restricted from doing certain things, and encouraged to do others. These things are reversed from the things good is encouraged to do. For example, evil is encouraged to be brutal and cruel, restricted from showing mercy. If you show mercy all the time, then another evil observer will say, "Hey Joe, you just aren't evil anymore. You are weak and merciful." That isn't to say that you can't ever show mercy as an evil character any more than a good character can't ever say, "Now you must pay for your crimes.", but there is a definate expectation within the ethics of evil that you will return your enemies every slight that they give you several times over. And, if you don't, you aren't adhering to the tenants of evil.
Likewise, in the ethics of evil there is the definate expectation that you will prey upon the weak and helpless - that that is exactly what the weak are there for. If you don't treat the weak as something to be trampled on, to be used as the raw building material by which you are constructing an ediface to the greater glory of evil, and if in fact you are actually being
benevolant rather than malevolant then you are off the right path and clearly not evil. Your role with respect to the weak is to show them that they are weak and useful only in so much as they serve there purpose. Your role isn't to help lift them up and give them a false belief in their own value and worth. As an evil character,
you are forbidden from doing that. That isn't to say that you can't use and manipulate people, but you do so only with the intent of knocking them down. You aren't ever practicing 'tough love' - love and all that crap is for the good guys.
Likewise, in the ethics of evil the only life you love is your own - and maybe not that. You can't value life as an inherent thing. If you are valuing life, practicing benevolence, and and being merciful you can't say, "Heh, I'm evil. I can do what I want, including being good.", because that's a ridiculous attempt to claim that good and evil are merely labels. Evil characters may believe that the only difference between good and evil is that good is a lie and a hypocricy, but they always believe that there is at least some important difference.
When players want an alignment that is "eh, whatever", they are really searching for neutrality. Incidently, nuetrality includes the belief that good and evil are merely labels and that there is nothing intrinsicly different between the two.