I think Debra's behaviour can be both explained and excused by her Crowning Moment of Awesome speech from the season 6 opener "The Angry Family:"
"Eileen, you have no idea what I have to put up with. When I got married I didn't just get a husband. I got an entire freak show that put up their tent right across the street. And that would be fine if they stayed there. But every day, *every* day. They drop a truckload of their insane family drek into my lap. How would you like to sit through two people in their sixties fighting over who invented the lawn? The lawn! And then the brother! [She starts imitating Robert] 'I live in an apartment. I don't even have a lawn. Raymond has a lawn.' [as herself again] But you can't blame him when you see who the mother is. She has this sick hold on the both of them! And the father is about as disgusting a creature as God has ever dropped on this planet. So no wonder the kid writes stories! I should be writing stories! My life is a gothic novel. And until you have lived in that house with all of them in there with you, day after day, week after week, year after friggin' year, then you are in no position to judge me!"
She is a saint who has been reduced to a harpy by severely extenuating circumstances. Enough said.
I guess we all need to remember the Rule of Funny. (http://tvtropes.org/pmwiki/pmwiki.php/Main/RuleOfFunny) Anything goes, if it gets a laugh - including cruelty and lack of character growth. It is a sitCOM after all! I forgive the writers for making Debra a bit of a harpy especially as time goes on - that show gave me so many laughs it has doubtless added years to my lifespan. Let's also not forget that Marie was funniest when she was being the most outrageously cruel or critical. It's just the way this show ran - it was the constant conflict transposed with underlying loyalty and love - this was the main dynamic of the show. I guess Debra NEEDED to be a harpy from time to time to serve the storyline. In any case - as far as I'm concerned, all is forgiven!
