Continuity mistake: The clock on Natalie's lab wall jumps forward almost half an hour during a 2-minute conversation with Nick. (00:27:25 - 00:29:05)
Continuity mistake: When Cal falls against the canister in the lab and accidentally releases the poison gas, the position of the big yellow lever on the gas pipe shifts back and forth by 90 degrees between takes. (00:34:25)
Deliberate mistake: At the end, a line apparently cut earlier creates slight confusion. Nick says to Nat, "When LaCroix asked you why?" (why save vampires). In the earlier scene as shown, the elusive LaCroix spoke only briefly to Nat in the lab, and never asked her that question. (00:42:20)
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.