She was often opposed, much to her chagrin, by the family of Charles and Caroline Ingalls, a household of much less means that nonetheless had the kind of respect and affection she thought she could buy her family, once seeking to have the town's humble church/schoolhouse built up at the price of being called the Olesonville Church. Suffice it to say, the Ingalls' clan came out on top in most conflicts, big and small, especially if it was personal. Once, the Ingallses even hosted Nels, who had finally left the shrewish Harriet out of frustration. While they reconciled, Harriet did not so much change as simply became more like herself as time went by.
It is not snobbery or gossip-mongering, etc. by themselves that place Harriet Oleson as a villain. On many occasions, her schemes disrupted families and even indirectly ended lives. A town newspaper she created was nothing more than a 19th Century tabloid, exposing 'secrets' of the townspeople and causing friends to turn on, suspect each other and not speak for fear of being reported on. Her gossipy side again showed itself when she set up and operated the town's first telephone network, using it to listen in on private conversations, till one frustrated husband literally tore his local pole out of the ground.
While instances of real humanity and pragmatism were not unknown for her, one incident in particular marked her for good as an uncaring manipulator. A girl that Albert Ingalls was seeing was attacked and raped. Out of fear of her cold and distant father's rage, she kept this a secret until she realized she was pregnant. Albert quickly decided that he loved this girl no matter what, but her refusal to name the father was the talk of the town, and Mrs. Oleson was quick to whip this up into a frenzy so fierce, that even when the girl told her father the truth, he would not believe that Albert himself was not the father, and forbid him to see her. Pleas to back off fell on deaf ears as Harriet enjoyed her perceived power and the attention her 'moral' stance brought. Albert even gave in and said he was the father, simply because his own family began to question this. When he finally angrily told the truth, disgusted even with his father Charles, he asserted that Harriet's rumor-spreading had given him no choice. But as they argued, the rapist again attacked the isolated young woman, this time killing her. Though he fled, he was mortally wounded. The girl Albert had planned to marry he instead helped bury, alongside a despondent father who felt his failure like a knife in the gut. While the rapist bore the ultimate responsibility, and the town including the Ingalls fell for a line, it can be said that a young woman and her child might well have lived if not for Harriet's inability to shut up. For all these incidents and more, she was never really called out in a way that got through her pride and ego.
In the show's final season (really a reboot/spinoff of sorts), Harriet was made miserable by the departure of Nellie, married to a man who moved them to New York, where she had twin boys. Nels gave in and they went to adopt a new child. The one they found, named Nancy, was physically almost Nellie's younger clone, and morally she was the concentration of the very worst aspects of her new sister at that age. No matter what Nancy did, including locking a teacher she didn't like in a freezer to die, Harriet kept right on indulging her literally to monstrous levels. A change in Nancy might have come later, but the show's new focus on Laura Ingalls and her husband Almanzo Wilder never caught on. The Olesons played a less prominent role in the final TV-Movies, with only Nels having a speaking role in "The Last Farewell" as the townsfolk destroyed their homes rather than see them fall into the hands of a greedy speculator who now owned the land. Willie and Nancy were seen in the background, but not Harriet.
Like similar irritants Ted Baxter on The Mary Tyler Moore Show and Frank Burns on M*A*S*H*, Harriet early on seemed capable of understanding her flaws and stepping up on occasion to overcome them. Also like those other characters, this version did not last long, and she like them became a constant headache. Baxter remained mostly an irritant on his show, while Harriet and Burns leave comic relief behind when one considers the lives they ruined.