Evil comes when the user is expected to lose significant amounts of something for no return.

Tobacco companies expect you to lose years of your life, tens of thousands of dollars and return you yellowing teeth and nails, smelling bad and lessened taste and smell. Evil.

WoW expects your monthly subscription fee and you'll probably put some time into it as well. They don't actually care how MUCH time you put into it, just that you're engaged enough to keep playing. In return they give you fun, a sense of achievement, a vibrant community and new content. Not Evil

Zynga games are boring. In fact they're worse, they're work. They guilt you into playing, they try to manipulate you into harassing your friends, they're designed to nickle-and-dime and get you playing as much as possible. In return you might get a sense of achievement, but you irritate your friends and waste a bunch of time on a game that isn't any fun. Evil.

I think it's always evil to engineer a product to be addictive. Compelling yes, addictive no. If you don't return a positive benefit to the user then even compelling is evil.

There are people who enjoy smoking and playing Zynga games, though. Positive benefit is relative. Just because you don't like cigarettes or Farmville doesn't mean that no one does. The argument that Blizzard doesn't care how much time you put into World of Warcraft doesn't hold up, either. The lower levels have a lot of grinding and the higher levels have multi-hour raids. World of Warcraft has addictive properties baked in to keep you playing, just like Zynga games. You could make the argument that Zynga is "evil" for milking addicted players for thousands of dollars, but Diablo III's auction house creates a similar dynamic. Isn't it enough to say that you think Zynga's games suck, but Blizzard's are good?

That's a very good and important question. It's too easy to dismiss it, because "it's just a game". Still, people do get addicted.