Eh, with me it's pretty simple. Evil is the one who does harm to others for no other reason than enjoyment. As much as I play with the idea of it with my characters, I think it's pretty clear-cut. Now here's where I find the line blurs.
People confuse morality with honor, in my opinion. Some would argue otherwise that there's no difference, but I think there's a huge difference. Most sociopaths even have some level of honor; you screw with their...well, not loved ones, but..family, then they will kill you. Honor is having your own principles, moral or immoral and following those principles.
Morality however is seeing whats right and wrong, which has nothing to do with principles. Honor might sometimes for some people require them to do something that isn't moral. Therefore this is where I think the argument of good and evil can be construed different ways.
But I think of it this way; Good is doing things that help those who do not wish harm upon others. Evil is doing harm to others for enjoyment.
In the end though, it's just my opinion. There's other ways to think of it, and there's nothing to say who is truly right or wrong. Everyone lives according to their own reality.