Alright, so I just finished 'Sirens' by someone (I think it's by K. Joseph? I always get the author mixed up), and that ending left me sitting there staring at the wall for a solid ten minutes. The whole book builds up this tense cat-and-mouse game between the detective, Finch, and the serial killer using nautical mythology. You're convinced the killer's gonna get away with it, right up until the final confrontation at the abandoned lighthouse.
Finch doesn't actually catch him in some epic shootout. Instead, he uses the killer's own obsession against him, quoting the same Siren's song lore that's been driving the murders. It triggers a kind of psychological break, and the killer just... walks off the edge of the pier into the stormy water. It's anticlimactic in a way, but also chillingly perfect. Finch wins by understanding the madness, not by outsmarting it with procedure. The resolution is bleak—the case is closed, but you're left wondering if Finch is now haunted by the same dark pull to the water. Last line is just him watching the waves, and it gave me chills.