I wouldn't call it "betrayal." That's a harsh word that suggests some deliberate act of malice or disregard. I think he was done in by hubris and a lack of self control. AFFC/ADWD were going to take a long time, I get that and don't think we can re-write that period of history or harp about what he "should" have done instead. What came afterward is another story. After finishing ADWD, Martin signed onto multiple deals and projects in a gust of optimism. I think that's where the hubris came in. He wanted to capitalize on his new fame, and bask in the positive reaction ADWD received (from the press). It was a victory lap, basically. I understand taking a victory lap after completing such a difficult book, but Martin took his eye completely off of his job. And now here we are in 2015 with no book.
Don't get me wrong, I'm not saying Martin should have immediately started writing TWOW after completing ADWD. But he spent two years doing nonstop events, editing anthologies, greenlighting projects, writing hundreds of pages of a book he doesn't plan on releasing anytime soon (the Targaryen history book), etc. That's solely on him. Now his magnum opus will be spoiled by a sup par television show.
I'd imagine our disappointment doesn't come close to his. He has to know he fucked up.