The fact that he can seem like a foot-stomping child is perhaps one of the things that makes him one of King's best characters -- he's so unpredictable that one moment he can seem like this and then the next he's decided to stab you or something similar. I think it's rather disconcerting when a villain can seem childlike and playful, rather than just constantly brooding and nasty. With those kinds of villains, you have
some idea where you stand; they are nasty, evil; you know you're not in for a pleasant time with them. But with Flagg, he can seem like your best friend, he can make you feel ten feet tall and the coolest person around; then suddenly he seems like the devil himself and will have you cowering and screaming.
Also, being childlike means that, as with small children, they don't fully understand the implications of what they are doing. They do things because they want to, because it's fun, because they are curious and don't really think of the consequences. Again, a villain who does that, who kills and maims and tortures because they are curious, because it's fun, is a lot more horrifying...
I have also heard that he apparently an incarnation of Nyarlathotep from Lovecraft's work; he embodies the Crawling Chaos. Of course, he has his flaws, and he is not an unbeatable force (
Dark Tower, munched on by a baby spider and brought about by his own arrogance) but again he induces chaos on a whim (it's been a while since I read
The Eyes of the Dragon but I think I recall that he wanted to throw the kingdom into a thousand years of chaos and anarchy...and is any reason given?) He is the chaos to the 'law and order' of the other characters in King's books: Peter and Thomas, Mother Abagail, Nick, et al, and of course, Roland.
So I guess this makes him seen a more intimidating character (at least to me): his unpredictable nature, his desire to spread chaos on a whim, etc. And he's also an admirable character because of these; and he certainly does have an intriguing, almost winning nature about him sometimes. As I said, sometimes he can seem like your best friend and at those times he seems charming, witty, intelligent. And who can't help being attracted to villains that have shades of grey...who doesn't find themselves secretly cheering for Iago now and then?