For Diana to come in and preach about how "wrong" it is they might have a reaction somewhat like this:
I believe that to think they would repent for those particular actions it would be a little out of character at a very basic level. The most realistic way of handling this is to let them own their actions and stand by them. I think it is also a little unrealistic (although OK to speculate about) that the younger ones don't know of these rituals. To hide it would be an admission of guilt, and I don't think they have (or should have) any for the reasons stated above.
But.. that's just me of course.
This is NOT to say that they don't have options. Such as infiltrating the outer world for a few nights and returning to Paradise Island without killing their mates. But these are not options we were presented with... so far.