Charlotte Lucas was not a cold woman, but she was calculating. She had calculated her chances of love at the advanced age of 27 at zero, and her chances of a second marriage proposal even lower than that, so that when Mr. Collins had made his offer she had not hesitated to accept.
She rather liked the rectory, and she could stand his prattling and didn’t really mind lying with him, but oh, that Lady Catherine de Bourgh — she could not stand listening to Mr. Collins speak of her in that tone of his. Charlotte calculated that she was likely to spend several days, cumulative, of her remaining life in Lady de Bourgh’s presence, which was bearable, and an addition incalculable number of her remaining waking hours listening to Mr. Collins speak of Lady de Bourgh, which was not.
So Charlotte calculated the swiftest path to freedom, which turned out to be Mr. Collins’s death.
Charlotte considered her options and chose poison, having calculated her chances of success the highest through Mr. Collins’s stomach. She served him elaborate desserts, appealing to his weakness, and he was unwell in days and gone within the month. There was no inquest. Everyone knew Charlotte Collins did not care for sweets, so it was not suspicious that she didn’t take dessert, and never widely considered that she could be her husband’s killer.
Charlotte’s old companion, Lizzy Bennett — now Lady Darcy — was the one whom, as always, Charlotte could rely on. Both of them now without the inheritance for which Mr. Bennett had failed to produce an heir, Lizzy (and Mr. Darcy) took Charlotte in, and she lived in great comfort in rooms at Pemberley; free to live as she pleased, she wrote novels about spinsters until the day she died.
She never once regretted killing her husband.