I really like some of the characters you mentioned, but not because they are dark... I think that liking a character only beacause he's "dark" is a dumb reason. It's like when someone likes a character because he has long hair...
I like well-developed characters, with good backgrounds and personalities, and I happen to like some sad histories....
Magus is great because of his history, raised by mystics, he became their hero, and a menace for humans... But he doesn't care about that, he only wants to find Schala, and lay his revenge upon Lavos. And he has a great personality, he's awesome...
Kain wasn't really dark, but I loved the way he's loyal to his good friend, but envies him for his position at the Red Wingss and for the love of Rosa... So he falls to the Dark side, it's great too...And Cecil was a villain, tricked by the King to do his evil deeds, used for his loyalty to Baron, but he regrets and fight against it all, that's what I call a good history.
Now Squall lacks everything they have. He's only a dumbass without a real purpose, he's "dark" only because it's "cool". Pfff...
