Yes, he's despicable because he preferred to write a story with realistic characters that shows just how dark humanity can be. There have been morally good characters in ASOIAF, they die very quickly because they are not willing to get their hands dirty by people who are willing. That's real life.
I disagree: I see it more as George R. R. Martin not wanting to cheat to let the protagonists win and likewise he does not want to cheat to have the antagonists win. This means both feel the consequences of their character flaws and actions.
Take the Red Wedding and how in large part it came about;
Robb Stark was willing to dirty his own hands. His death was the fatal result of his flaw of honor: he felt honor bound to marry a woman whose virginity he'd taken when she consoled him about the death of his brothers. This lead to Walter Frey feeling spat on as Robb Stark had promised to marry a woman of Frey's house. As revenge, Walter Frey arranged the Red Wedding leading to the death of Robb Stark.
This pettiness was in turn the flaw of Walter Frey. As a consequence of what he'd done in betraying Robb Stark and breaking the rule of sacred hospility, people began targeting members of his house which had it's reputation utterly ruined. The exact opposite of what Walter Frey wanted had happened as now no one respects the Frey house and now even more people than before spits on it and considers it beneath them.