I've had cases where the lab simply cannot make a conclusion. The blood can get mixed with other things and deteriorate over time. On TV, the DNA evidence always comes back.

DNA can't prove intent. You have cases where you already know who did it. The question is, when you take all the facts, what crime is that? First-degree murder? Second-degree? Manslaughter?

Most cases don't turn on DNA evidence. Most turn on eyewitness testimony and admissions by defendants.