Revealing mistake: When Natalie shows Nick the murdered museum guard's bite wounds in the morgue, the "dead" man's carotid artery is pulsing quite healthily. (00:08:30)
Factual error: The murderer works for a blood bank and drives a bloodmobile, a plot holdover from the first pilot ("Nick Knight"), which was set in Los Angeles. This second pilot, however, is set in Toronto - and Canada doesn't have mobile blood units. (00:31:35)
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.