Between Mary Carson and Luke O'Neill, who is the worse character in "The Thorn Birds"?

Interesting question! Definitely, the answer to your question will be based on your own analysis and judgement about the two characters. Nevertheless, I'd like to put forth the hypothesis that, of the two, Mary Carson is by far the worse villain.

While both Mary Carson and Luke O'Neill are characters who lust for power and passion, it is Mary Carson who seeks to deny another the passion she most cherishes, but can never have. In the novel, Mary becomes infatuated with Father Ralph. The good priest, as we know, seems to be preoccupied with only one thing: his lust to ascend the hierarchy of the Catholic Church.  He thinks that Mary Carson is the answer to his ambitious prayers. However, Mary has plans of her own. She plies the priest with material privileges in order to seduce him.

Father Ralph is free to move about Drogheda like a young prince; his horses and even his car are gifts from Mary, the matriarch who rules Drogheda with "imperial malevolence." This phrase foreshadows what a formidable nemesis Mary can be, especially when her desires have been thwarted. Despite her age and her claims to be beyond the desires of the flesh, Mary is jealous of Father Ralph's fascination with Meggie. In her youth, Mary had been beautiful, and her indulgent husband had doted upon her while he lived. Now, in her twilight years, the thought of a man under her spell still energizes her.

Unfortunately for Mary, the inimitable Father Ralph isn't interested. Humiliated by this, Mary promises Father before her death that she will make him "writhe" in agony and that he will sell himself like "any painted whore" for the chance to fulfill his priestly ambitions. And so, she fulfills her promise. At her death, a separate will is disclosed to Father Ralph, where Mary leaves her entire fortune of thirteen million pounds to the Catholic Church on the condition that Father Ralph is to be the executor of her estate and as long as the Church "appreciates the worth and ability of the said Father Ralph de Bricassart."

With this masterstroke, Mary's "imperial malevolence" obliterates any true contentment from Father Ralph's life. Father Ralph decides on the side of his ambitions, and the result, as you can see, is disastrous. He never knows true peace and joy again.

As for Luke O'Neill, he is certainly a reprehensible character. As ambitious as he is lustful, he marries Meggie for her beauty and her wealth. After their marriage, Luke appropriates all of Meggie's pocket money as well as her monthly income. He is rough and brutal when the couple finally consummate their marriage; to make matters worse, he is also a miser. He denies Meggie the comforts any wife has a right to expect in the forbidding and unforgiving terrain of the Australian outback. After a brief honeymoon, Luke engages Meggie to work for the Muellers as a live-in housekeeper, while he travels far away to work in the sugarcane fields with a group of cane-cutters.

When Justine is born, Luke ignores his daughter and refuses to see her. For the rest of the story, Luke is indifferent to Justine. As Meggie tells Anne Mueller, Luke is the kind of man who despises soft and pretty things. He would rather spend his time with strong and independent men like himself and roam from one end of the earth to the other. Luke is completely self-absorbed and oblivious to the needs of his wife and child; because of his callousness, Meggie eventually leaves him and returns to Drogheda.

For all his despicable characteristics, however, Luke isn't vindictive like Mary. He does not seek revenge on Meggie after she leaves him; after all, Meggie does let him keep the twenty thousand pounds she brought into the marriage. Instead, shall we say that Luke is so entirely ignorant about the responsibilities of a husband that he is more of a fool than a vengeful character? Bear in mind that, after their separation, Luke actually sends a letter to Meggie to reassure her that he has no intention of signing up to fight in the war. According to the text, Luke doesn't want Meggie to worry about him.

Of course, Meggie is incredulous at her estranged husband's distorted perspective. In due time, Luke writes a second letter to Meggie, suggesting that she may want to return to him now that he is making a steady income from cane-cutting. So, Luke is definitely oblivious to his insensitivity, but he is far from a vindictive character like Mary.

Hope this helps!