I wouldn't really say dark. Sure, it has dark story, violence and nudity but you can find such scenes in any R anime, Monster is dark when it comes to human nature core and how dark those human nature can be, the despair, the dark questions in the anime and the dark philosophy. Well once you watch it and try understanding it deeply you'll know how dark it is, not violence but words themselves.
The remake is actually not popular @SoonOFF