@DeadParrot
And they are always excellent points as well, that's why you get all tha karmaz.
@Fifths
It's really interesting when you think about it, because on the one hand, you have Fluttershy, upon whom we seemingly had forced hypnosis. On the other, you have Rainbow Dash, who had a very real dilemma to consider, and it doesn't take a huge stretch of the imagination to realize that she may have made that choice very consciously. You also have Rarity, who actually made a distinct effort to resist Discord's influence, but unfortunately lacked persistence in that effort. And then, you even have Twilight, who managed to come out from under the hypnosis without the particular aid of magic at all. It really does create an interesting scenario where conflicting desires and strength of will all seem to play a part.
Yes, magic was involved, but that wasn't the entire picture. Otherwise, how would Twilight have managed to break out of it without some kind of spell? A contest of wills was definitely at play.
As far as the notion of cruelty goes, he tore apart the closest of friends and seemed to take great delight in doing so. If that's not the very definition of mean-spirited, I don't know what is. I mean, just look:
Here he is, having created an unwinnable situation for Dash where she literally has to choose which friends she's going to protect from his antagonism. And he couldn't be happier about it. He's the type of character that enjoys the suffering of others above all else; and if he's not cruel, then neither is the Joker for cryin' out loud. Realize that nature can't be happy, or unsettled, or experience any other kind of emotion, but Discord displays these attributes, and is quite capable of reason. Rather than behaving like a force of nature, I see him as the pinnacle of cognitive evil.
As for Fluttershy, I think it does speak loudly to her emotional stability at the time that she simply didn't possess any of the emotional angst that he was looking for.
@The Sims Analogy, below
The difference being that you know full well you're not toying with living, breathing, feeling creatures. Discord has no such excuse.