Lady Bellaston's part in Tom Jones is, frankly, a little bizarre. She is a relative of Sophia's, and Sophia runs to her house in London when she is first trying to escape her father and Mr. Blifil. She believes that Lady Bellaston will take her in, because Lady Bellaston has teased Sophia in the past about being too obedient to her father. Even before we meet her in person, the fact that Lady Bellaston doesn't believe in fatherly authority is a sign that there is something potentially morally wrong with her: she thinks Sophia is a simple country girl for being nice to her dad.
And once we do meet Lady Bellaston in the novel, we realize that there is something really off about her (at least, according to the social rules that Tom Jones appears to support). After all, she responds to Tom's apparent marriage proposal by writing back to say that she is offended by the suggestion that she should unite her fortunes to some man. But the final resolution of this novel is Tom and Sophia's wedding. The fact that Lady Bellaston automatically turns her back on a social institution that the book appears to value so much gives us more proof that she is a deeply selfish, disrespectful, unfeeling person.
Well, it's not exactly that Lady Bellaston is unfeeling. It's just that she is never, ever honest about her emotions. She has feelings, but she keeps them deeply under wraps; her motivations are never clear to the people around her. And again, as is the case with Mr. Blifil, since Fielding clearly despises liars, her dishonesty makes her one of the worst villains of the novel. (Speaking of lies, we include some thoughts on Lady Bellaston's deceptive appearance in our section on "Character Clues.")
As evidence of Lady Bellaston's two-faced behavior, let's look at her first real appearance in Tom Jones. Mrs. Fitzpatrick goes to tell her the whole Tom-Sophia saga, because Mrs. Fitzpatrick wants Lady Bellaston's help to keep Sophia away from him.
When Lady Bellaston hears the details of her relative's crazy romantic life, her mind doesn't jump to sympathy or plans for Sophia's future. Instead, she engineers a meeting between Tom and herself so that she can seduce him. The two of them do the deed, and Lady Bellaston starts showering Tom with gifts and money. She basically keeps him as her own personal (paid) sex slave throughout most of Tom's stay in London, even though she knows how badly Sophia wants to see him.
Honestly, we don't necessarily have a problem with Lady Bellaston maintains a swinging bachelorette pad for her hook-ups. The central issue here is not Lady Bellaston's sex life. The problem is that she sees nothing wrong with going to truly extreme measures to get what she wants. She has also made such a habit of keeping secrets that she actually keeps a hidden apartment—now that's dedication to deception.
Lady Bellaston freely uses her wealth and social standing to cover for her frequent affairs. Since she is so used to acting in secret and without any kind of oversight or reality-checking, she stops at nothing to get her way. We see proof of this no-holds-barred, take-no-prisoners approach to getting what she wants when Lady Bellaston manipulates Lord Fellamar into (a) attempting to rape Sophia, and (b) trying to kidnap Tom and force him into the navy. She really has no limits when she's trying to get what she wants.
So, we have tons of evidence that Lady Bellaston is messed up. But the thing we find honestly weird about her role as a villain in the novel is that she never pays for her wrongdoing. Tom and Sophia go off to get married and Lady Bellaston treats Tom as though she has never met him before. That is the extent of her payback for manipulating Lord Fellamar and Tom.
Squire Allworthy can punish Mr. Blifil because he is under Squire Allworthy's authority. But Lady Bellaston is an independent woman; there is no one to say that she shouldn't go about her business as usual. She may have a slightly worse reputation, and she may be a bit unhappy about the loss of Tom, but none of that is anything like as dire as what Mr. Blifil suffers, with his humiliation and his loss of his inheritance.
And this lack of any kind of real payback is what makes Lady Bellaston interesting to us. She appears to be part and parcel of the social world in London. Her dishonesty is just part of the scene in the city—for example, we can compare her behavior with that of the Man of the Hill's London friend, who sees no real problem with cheating and conning other people.
Consider the liar's club to which Lady Bellaston belongs: the members of this "comical society" (15.3.4) are supposed to tell a lie every day and then laugh with each other about the results of these fibs. In other words, this whole association of scoundrels runs around London telling lies just for the sake of it, to amuse one another.
So Fielding implies that hypocrisy and deception are actually the rules of London high society rather than the exception. Lady Bellaston doesn't have to suffer much payback because nothing she does seems too far outside of the London norm. The characters who participate in high society life in this novel (including Mrs. Fitzpatrick and Lady Bellaston's friends) appear to be pretty amoral in general, meaning that they just don't care that much about whether things are right or wrong.
We would never say that Lady Bellaston is not a villain. Clearly, the book sets her up as a lying, manipulative monster. Nonetheless, we feel like Lady Bellaston has some traits worth admiring. She is an independent woman in charge of her own fortune and her own sexual destiny. She has much more self-determination and freedom than Sophia has, even though Sophia is our heroine and Lady Bellaston is a villain.
We feel like at least part of Lady Bellaston's negative portrayal might come from Fielding's conservative gender politics. The extreme evil of her characterization could be at least partly because she is a strong woman character in a social system that otherwise favors masculine control of money and power.