How does Ophelia think slaveholders ought to treat their slaves? Why?

From the text:

"I think you slaveholders have an awful responsibility upon you," said Miss Ophelia. "I wouldn't have it, for a thousand worlds. You ought to educate your slaves, and treat them like reasonable creatures, -- like immortal creatures, that you've got to stand

before the bar of God with. That's my mind," said the good lady, breaking suddenly out with a tide of zeal that had been gaining strength in her mind all the morning."