I see it this way -- evil has a goal and will do anything to acheive that goal, Need to seige a city and kill everyone inside? Evil does it. Need a super weapon and the only person who can create the weapons don't want to have anything to do with you? Grab his daughter and give him the orders. A little old lady between you and a fortune? A shove down the stairs.
In short, Evil will do anything it can to do its thing. Most heroes have some sort of line they won't step over. Evil had only the restrictions that have been palced upon by someone with grater power than them and even then, Evil is looking for loopholes (Reason why Lawyers are mistaken for being all evil (Actually only 99% of lawyers are evil -- the other 1% are shoved into political office where they can do little harm...))
Inshort, evil is power without much restraint (For the most part) Good have some rules to follow, even if they are flexable...
Craig