What he is doing is trying to show the crime that actually was committed is impossible. The fact that it happened shows that its possible. My take: What he is doing is trying to show that the crime, as theorized by the prosecution, is impossible/not plausible.
He started out saying maybe she was killed in her bed. Her blood was in that room. And DW did not know how to get into that house or what room to go to. All fingerprints excluded DW.