In short:
Good neo-noir crime mystery somewhere between Monk, The Sixth Sense and Asian hard-boiled movies like The Chaser.


----- Random thoughts:

Mad Detective is not as violent as say "I Saw The Devil" except for one gory scene at the very start. It's a weird scene, and doesn't quite fit the tone of the rest of the movie.

There aren't many action sequences, but they're okay.

As a Westerner, telling the young male actors from each other was hard at times.

Even though they're very different movies, I'd like to compare Mad Detective to The Wailing. They both attempt to pull the same trick: keeping you guessing what's what until the very end, and make you change your guess multiple times on the way.

The Wailing, being the masterpiece that it is, succeeds way better at doing that. Plus it has mesmerizing performances by at least 2 of the protagonists.

----- Verdict:
Good (7/10)