Plot hole: Nick calls Schanke and asks him to take the Polaroids in to a lab for analysis. But Nick seems to have forgotten that he has the photos with him. They're not back at the precinct station for Schanke to take them in. (00:30:25)
Plot hole: The insertion of a musical sequence used to fill extra time in this episode creates a plot problem. It's spliced in between Nick's urgent plea for Nat to stall the autopsy and his rescuing the bound and gagged Rebecca, and unfortunately makes it appear that in that interval, Nick simply went home, played music and sat in his loft window brooding, ignoring the case altogether. (00:40:40)
Answer: Nick was sick and tired of being an immortal bloodsucker. He wanted to be human, fall in love, get married, have children, grow old and die. As for Janette, according to her, she fell in love and the passion she felt "cured" her of her blood lust.