Many compared Through the Darkness to the US series Mindhunter. Though they share some elements, they are completely different shows. One focuses on the mind of serial killers. This one focuses on the psychology of profilers.
They could not have found a better actor to portray an empath. And they couldn't have written a better script to profile the empath archetype: quiet, introverted, loner with few social connections. It's downright eerie how well the empath type was depicted. This personality type takes every emotion they're exposed to onto themselves. If they're not careful, their burden will consume them.
By contrast, the serial killers' stories were deliberately made crass and vulgar to underscore the banality of evil.
If you're going into this with a 'Korean Mindhunter' mindset, you'll likely be disappointed, like so many other commenters. But if you like intense psychological exploration, this is the show for you.