Death is rather depressing.
Therefore, considering what we already know about the Castle season finale, it's no surprise that producer Andrew Marlowe has labeled the episode as "dark" in a recent interview with Entertainment Weekly.
“It was less of a decision to go dark and more of a decision to deal with Beckett’s mother’s murder, which of course makes her emotionally raw and means we have to take everything about the case very seriously," Marlowe says. "It always challenges the Beckett/Castle relationship in terms of drawing them together or pushing them apart.”
Will the May 16 installment conclude on a cliffhanger? Yes, the producer teases, adding it will center on both a case and the Beckett/Castle relationship.