I think this is a great topic, and I had a nice, long, explorative post all typed and ready to send, but the internet ate it. So I'm going to attempt to recreate it.
Judith's behavior, as well as that of her three siblings, I think, is a result of their lack of parenting. As someone mentioned previously, they were raised by Edith, who was two generations their senior, and who probably had some very different values and operated under the "children are seen and not heard" ideal. Edith and Gabriel, if we can assume they are the same ones we saw in 1840, were not suited to be parents in the first place, since we learned that they sent their children away to boarding school.
The resulting adults that we view in 1897, Judith, Edward, Carl, and Quentin, are products of their upbringing, or lack thereof, and, I believe, act the way they do as a result of competition between themselves for attention from Edith and Gabriel. Again, assuming they are the same couple we witnessed in 1840, we know that Gabriel was concerned only with getting out of Quentin's shadow and proving himself to his father, and that Edith was so disloyal and flighty that its a wonder she could've lived (oops) to be as old as she was and still reside at Collinwood. Much of the time they probably paid no attention to their grandchildren, because they were too concerned with themselves, and plotting against each other.
All the children, as a result of this, probably competed for attention from them, craving some type of parental guidance and care. The rivalry that was spawned speaks to why none of them get along when they are older, and, we hope, wiser.
Judith may have attempted to win affection from Edith by becoming a young matriarch. She made it her mission to learn everything about running the house, believing that this was what a good woman would do, and what her grandmother would want to see from her. But trying to impress Edith by doing all these things most definitely would've failed, and Edith may have even chastised Judith for her attempts (given that Edith never really ran the household anyway). Over time Judith became hardened, seeing that nothing she did would gain her respect or attention from her grandmother. Because of her constant attempts to impress her mother figure, she had no time for relationships, which led to her being very naive regarding love and marriage. Judith being a religious woman, or at least beliving and God, and thinking that donating to Trask's school might impress someone (God, maybe?) she offered to rebuild his school, and then he subsequently professed his (false) love for her, and because she had never been in a serious relationship, and was naive in that department, not to mention that she was so taken that someone was going to finally pay attention to her, that she fell quickly.
The other possibility is that once she realized that Edith was never going to take care of the children, she stepped up to the plate and the result of having to be a mother to three unappreciative children made her cold and hard.
Edward, meanwhile, cast himself as an upper class, stiff-upper-lippy guy, who was concerned only with protecting the family name and doing well in business, all in the name of gaining respect from his father figure, Gabriel. When none of this worked, Edward upped his game until he became the stuck-up, ambivalent guy that he is.
Carl took the usual road that kids take--being obnoxious, loud, and pranking in order to get attention. Because he wasn't parented and told not to do these things, he thought they were okay, so he contined to do them even into adulthood, because they were his only source of comfort and pleasure, and the only way anyone would ever pay attention to him.
Quentin recognized his natural ability to charm people at a young age, and put his charm to work, attempting to get attention in this way from his grandparents. After years of trying different approaches, his charm now perfected, and having realized that he would never get anything out of those two old bitties, he turned to womanizing to feel something, anything at all. Adding booze to that only increased the pleasure, and the young playboy that we saw in 1897 was born.
Now, of course, none of this is based on anything but what we saw in the series, and all of this is mere speculation on my part. But I think that its not too far-fetched to see origins of these people in the way that they act as adults, because if they had attention from their grandparents they wouldn't have tried to appeal to the ideals that they thought their grandparents had.
But, it's always possible that Gabriel and Edith DID have a hand in these children's upbringings, and that they taught each one of them a golden aspect of their personalities, whether it be lying, cheating, stealing, drinking, womanizing, backstabbing, etc.