For me, the series only had 2 missteps of note. (1) I thought Kouichi's reaction to the possibility that Teshigawara had possibly killed his friend was too off the mark. Kouichi's "let's go check on him" had no more urgency about it than if he was going to check on the weather outside. This odd reaction continued on through his entire circuit through the mansion. (2) The way everyone basically ignored the fire. Yes, TV & movies seldom depict the reality of a burning building. But at least their characters usually react to being inside a burning building. Our students, on the other hand? Barely noticed it at all. They should have first escaped the building and then went on their killing spree. Self-preservation and then murderous intent. Get your priorities straight.
But you know what, despite the missteps of the last 2 episodes, overall the series was a great ride. And since this is the overall series rating thread, I give the show a 9.