To make her novel believable, Stowe had to present a variety of characters that corresponded to the real kinds of people involved in all the aspects of slavery in pre-Civil War America. Not everyone has to be pure evil, like Simon Legree, for slavery to destroy the lives of human beings. Even people who supposedly mean well contribute to the problem.
Tom's first owner, Shelby, isn't a bad person as far slaveowners go. What Stowe wants to show is that even if a slaveholder is reasonable, the slaves are still at his mercy. After making promises to the contrary, Shelby still sells slaves, including Tom.
St. Clair is about as good as it gets as a slaveowner. He doesn't physically mistreat his slaves and gives them a relaxed atmosphere to live in. He owns slaves but doesn't approve of it. He has plans to free his slaves, but is killed before he does it. Stowe uses him something like Shelby, some owners might be the exception to the rule, but slaves are still subject to the whims of others. He is also an example of someone who tries to do good, knows he is wrong, but doesn't do anything about it.
With his wife Marie, who sells Tom after St. Clair dies, we see the negative effect of St. Clair's inaction. She sells Tom, ignoring the fact the her husband had planned to free him.
Miss Ophelia is a northerner and does not approve of slavery, but she still doesn't think much of black people. This character allows Stowe to show that even people who don't support slavery have negative attitudes about the slaves.