Basil says that he was out with his girlfriend until 9pm to which Rose responds, “That’s a whole hour unaccounted for…”

Well, how does Rose know that it’s one hour unaccounted for? If he was with his girlfriend until 9pm, then from 9pm to midnight is all accounted for…

But one only needs to account for the whereabouts until the victim is killed. Rose knows it’s an hour unaccounted for because she knows he died at 10pm. But the autopsy only just came in, so she couldn’t have known, unless…

Rose must have staged a robbery to cover her tracks! And so… Rose is the murderer!