Dark Knight: The Second Chapter (2) - S1-E3
Continuity mistake: The blood stains on Nick's white turtleneck are lighter, then darker, then lighter again, and migrate to different positions, both during his fight with LaCroix in the abattoir, and throughout the hospital scenes afterward. (00:10:15 - 00:19:30)
Continuity mistake: Schanke calls Nick to tell him they've found Mrs. Hedges' body, and says, "They cut her throat, man." We even see a brief shot of the bloodied corpse. But later in the morgue, Nick says, "She was asphyxiated, right?" and Nat agrees. The corpse on the autopsy table now appears to have an intact throat. So, which way did Mrs. Hedges die? (00:22:15 - 00:38:10)
Spin Doctor - S1-E15
Continuity mistake: Nick takes to the air wearing a blue windbreaker, but lands wearing a long black duster coat. (00:14:05)
Continuity mistake: Nick's beard is fully grown in the beginning of this episode, but it changes, disappearing entirely more than once and then reappearing in various stages of growth, even in sequences supposedly occurring on the same night.
Continuity mistake: While he's in the confessional, Schanke's raincoat collar is turned up. When he rushes out to confront Nick, it's folded neatly down again. (00:37:40)
Continuity mistake: Elizabeth the leper is wearing a dirty gray dress under her black cloak - until Nick makes her a vampire, when suddenly there's a beautiful, spotlessly clean white gown under her cloak. (00:09:30 - 00:26:30)
Continuity mistake: Nick gets a phone call telling him another murder victim has been found and that she's been dead for at least a week. In the very next scene, Schanke says that the victim "disappeared after a date last night." (00:27:00)
Continuity mistake: When Nick breaks into the old Chinese herbalist's warehouse, he twists and breaks a doorknob lock. But when he walks through to the inside, there's a latch-and-bar array instead of a knob. It's a completely different door.
Continuity mistake: The wooden plank with which Alexandra impales Nick completely changes angles (from flat side tilted left to flat side tilted right) between the time it strikes him and the time he pulls it out. It also appears to "grow" an inch or so wider than it was before. (00:39:00 - 00:40:25)
Continuity mistake: The tow truck and the red van it's towing change positions at the alley's entrance between shots. (00:01:00 - 00:23:35)
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.