So the child reader is supposed to get all this, but Harry, Ron, and Hermione do not?

Rowling is a bad enough writer that I can well believe it will turn out in book 7 that Harry totally misinterpreted the murderer's scowl of revulsion and hatred just before the event.

I wouldn't buy it, and I don't buy the connection between the murder and the poison either. That was some time earlier; he had ceased being in quite such a state of extremis.