What event leads to Augustine seriously discussing the issue of slavery?

In Harriet Beecher Stowe's Uncle Tom's Cabin, Augustine St. Clare is Tom's third owner, a man who believes that slavery is morally and philosophically wrong but nevertheless relies on slavery to sustain his own comfort.
Augustine buys Tom after Tom rescues his daughter, Eva (Evangeline St. Clare), from drowning. Augustine takes Tom back to his home in New Orleans and eventually entrusts him with the responsibility of doing the marketing for the family.

St. Clare at first employed him occasionally; but, struck with his soundness of mind and good business capacity, he confided in him more and more, till gradually all the marketing and providing for the family were intrusted to him.

As Eva lays dying, she entreats her father to free Tom and to become an abolitionist, which he agrees to do.

"Papa, isn’t there any way to have all slaves made free?”
“That’s a difficult question, dearest. There’s no doubt that this way is a very bad one; a great many people think so; I do myself I heartily wish that there were not a slave in the land; but, then, I don’t know what is to be done about it!”
“Papa, you are such a good man, and so noble, and kind, and you always have a way of saying things that is so pleasant, couldn’t you go all round and try to persuade people to do right about this? When I am dead, papa, then you will think of me, and do it for my sake. I would do it, if I could.” . . .
“There, there, darling,” said St. Clare, soothingly; “only don’t distress yourself, don’t talk of dying, and I will do anything you wish.


“And promise me, dear father, that Tom shall have his freedom as soon as”—she stopped, and said, in a hesitating tone—“I am gone!”
“Yes, dear, I will do anything in the world,—anything you could ask me to.”

A few weeks after Eva's death, Augustine discusses slavery and emancipation with Miss Ophelia.

“And what are you going to do?”
“My duty, I hope, to the poor and lowly, as fast as I find it out,” said St. Clare, “beginning with my own servants, for whom I have yet done nothing; and, perhaps, at some future day, it may appear that I can do something for a whole class; something to save my country from the disgrace of that false position in which she now stands before all civilized nations.”
“Do you suppose it possible that a nation ever will voluntarily emancipate?” said Miss Ophelia.
“I don’t know,” said St. Clare.

Augustine starts the legal proceedings to emancipate Tom, but he neglects the matter, and the emancipation process drags on. Unfortunately, Augustine is stabbed trying to prevent a fight between two men in a cafe, and he dies before he can free Tom or any of his other slaves.
Augustine's wife, Marie, decides to sell the slaves, and Tom and fifteen-year-old Emmeline are sold to Simon Legree, an unremittingly evil man.