Oy, this is getting really philosophical

I'd even go so far and say: Only when you mean harm to pretty much every person that you don't have a need for (or care for), you're evil. Like the classic action flick villain.

That's why it's hard to find a real human being I'd call evil. Bin Laden may try to kill every US American he encounters, but could be a loving father, I dunno. He'd still come closest, I guess.
A serial killer too. Then again, he could be a nice person to everyone but blonde hookers, who knows? Things aren't as black and white in the real world.

Martin's books are realistic enough to have good characters with dark sides and vice versa. But Cersei comes closest to being evil. She cares for her kids too, but means harm to pretty much everyone standing in her way to power.