Alright, this novel has one of my least favorite tropes. Nice-guy assassin. Those two tropes just don't work well together.
First off, the main character. The best assassin from a fantasy world, can even kill Kings and Queens. Is almost caught on his very first job. He is supposedly a assassin but in reality hes just a vigilante.
Side characters. Another review basically talks about this perfectly. All the characters are template characters. Cop who relies on intuition, girl whos a battle maniac, cute little sister, grumpy older brother, super-genius kid who can hack into anything.
Plot & writing quality. First off, this novel definitely has a amateurish tinge to it, but seems somewhat refined. The plot, however, seems super forced and is rather boring. Gang members beat up the main character's brother, he beats them up. They come back, with more, because? (This is a world with martial artists, why are these guys so keen on killing themselves) Anyway, he beats them up, takes pity on them, once again because? He decides to kill all the gangs nearby because of some pity story the guy told him. Later, when a cop invited him to dinner a young master decided to beat him up. He beat them up instead, and now decided to destroy his entire company and family. Good ol' vigilantism. It just seems rather forced, especially that young master part. (Again, there are literally martial artists in this world, the young master doesn't have a single martial artist)
Anyway, another gripe I have with this novel is "intuition". God it's annoying. A lot of novels go with this really forced way of advancing the plot. A random copy intuitively knows that the main character somehow committed the crime of killing gang members, so she invites him to dinner. A random battle-junky intuitively knows that the MC is the one who fought her before. (he was completely covered in black, no resemblance)
Overall, a amateurish novel that is rather low quality in general.