Your comment about how evil certain characters are, cartoonishly so, made me wonder, what is the point of such disgusting characters? A number of people have said how overboard the suffering of Jeyne and Theon is. Do you think there is value there or would what GRRM is saying be as effectively conveyed with a little less?
I think the biggest point is to make it overboard, honestly. Let's face it, after ACOK making Theon in any way sympathetic would have been. Difficult. (Although GRRM has expressed surprise bordering on shock that people, especially women, would like him or similar characters, such as Cujo Come Again or J@ime.) He was making a point over vengeance versus justice, as he often does.
We might have wanted Theon to suffer over betraying Robb, over Winterfell, over trying to murder Bran and Rickon, killing several side characters and murdering the miller's boys. But just as he apparently wanted us to feel some measure of sympathy during Joffrey's death, tried to remind us he was a thirteen-year-old boy and IRL might have just been a schoolyard bully, the point is that vengeance is not justice, and almost always goes too far. Theon had to be dragged through utter hell, so there was no doubt left with the readership that the author does not endorse this sort of punishment at all.
What he put Jeyne through is, however, far past acceptable. The worst we've seen her do is name-calling, for all the Ship Girls call her a bully. And here's the damn problem, the fandom. Jeyne is thirteen, had her father murdered by the Lannister guards, was sex trafficked and 'trained' in one of Littlefinger's brothels, and is now living an absolute nightmare complete with losing her sense of self. When half of the HEROIC main characters (so, not Tyri0n or D@ny) do worse on the regular than what Jeyne did, there is no excuse for her treatment. Except people somehow see this little girl, who would be in what, seventh or eight grade, and think she deserves this torture because she was mean years ago to their favorite character. I genuinely can't handle those people, they're messed up in the head.
Something else that may be the reason is that if he creates these cartoonishly evil characters, it actually helps hide some of his real villains - read: the non-heroic examples above. If they seem like they are given justification and complexity, they can't be the bad people, right?
The other bad guys are either literal zombies or flat-out evil characters like the Boltons or House (Jof-)Frey of the (Lannister) Twins. (Never let it be said George isn't a gigantic troll.) Cersei is the rare obvious villain POV, most others are written through secondaries (Stannis and Melisandre through Davos (and Stannis is largely hidden behind Davos' loyalty), Cressen, and Jon and Sam; Joffrey through a billion people...) or hidden through POV trap, or equally cartoonish and one-dimensional to offset the real villain being a POV character (the slavers in Essos to hide D@ny). The few times an obvious villain is made a POV, such as Cersei in FeastDance, Melisandre's lone chapter, or the Greyjoys, it happened books later and while George may flesh out their inner motivation some, they can still come out rather cheesy movie villain like.
All in all, they do serve several purposes. They help disguise the villainy of the big bad main characters by making them 'a lighter grey' on the sliding scale of morality in comparison; they are the ideal way to demonstrate how justice and vengeance are different things; and if he gives the Token Threshold Bad Guys 0 redeeming qualities he can give more care to his main villains and flesh them out better, since their tragic backstories and moments of goodness stand out more in the sea of rotten evil if these are rare examples.
That doesn't mean he doesn't go overboard or that it's not a revolting read sometimes. Jeyne was way past normal, and she isn't the only example.
Post Referenced: Not All Banners Are Grey at Night. Some Are Pitch Black