LAUREN Giddings' accused killer first became a person of interest, when he appeared distraught in various media appearances, cops told the court yesterday. Then they took a closer look and found the evidence they think will convict 25-year-old law graduate Stephen Mark McDaniel. Among his possessions: a master key to an apartment where Lauren Giddings, 27, lived next door and packaging for a hacksaw. MORE ON HOW COPS NABBED KILLER AT CRIME SCENE USA