I've read this book several times and love it.
Unless you speak directly to the author or perhaps read an intrepretation about the book by the author - can you really know what the author was meaning 100%.
I can see both being applicable and not necessarily conflicting. "Sin" or doing wrong does have consequences and it certainly shows this in the book - also not judging others makes sense as you can see as you get to know the characters and understand them - it isn't so black and white.
Maybe one class is focusing on one aspect of it, while the other is focusing on another. I always thought it was showing how historical we have treated people that committed a sin, but perhaps we should learn from this and look deeper at the situation. I mean poor Hester being married to that grumpy old man that treated her so horribly - how could you blame him - her hubby wasn't so innocent himself.