Fallen (1998) is a supernatural cop thriller, where Denzel Washington plays a cop who eventually realizes that his serial killer nemesis is actually a body-hopping fallen angel. Which means a few things: even when he thinks the game is over, there's still more to go; and anyone can be his enemy.
This gives Fallen a very 90s feel, by which I mean that it has a real sense of paranoia and urban uncertainty. What was with the 90s? Fallen also has a curious genre position, with hints of horror (the monster is going to get you!), procedural thriller (let me research these police records for the answer!), and even what we'd call urban fantasy today (since there's this whole cosmology of good and evil that the cop stumbles into).
As a script, the movie has some nice structure/arc in the hero's journey from a false position of happiness--the hero cop watching the execution of his major enemy; all the way to his real low point, when his brother has been murdered and all the cops who used to respect him now hunting him.
However, there's something a little cheap in using this fallen angel as the antagonist, since you don't need to worry about the antagonist's motives. Why does the fallen angel kill? Because he's a fallen angel. Why is he targeting the hero? Because he's a fallen angel. Why does he give clues that help the hero find out what he is? Because he's a fallen hero.
Now, there is a thin thematic note here, in the idea that the fallen angels want to destroy civilization; and the idea that the cops are the chosen people who protect civilization. But, eh, that never really says why the fallen angel targets this particular cop, or why the fallen angel's method of destroying civilization is... serial murder. Why not join the banking industry and help destroy civilization that way? But that would transform this 90s supernatural thriller into an 00 supernatural thriller.