This drama is fabulous, as a fan of thrillers I approve of it very easily. But it could have some modifications (only a little, but a little is still important).
Let’s break the aspects down one-by-one;

Story: The story was creative, although the core plot is not new. I mean we all have seen one such story before where a cop goes undercover to join the criminals and ends up sympathizing for them. But in ‘The Worst of Evil’ however, it doesn’t quite follow the same plot. it’s unique in its own way. Overall it’s great. I have one question though; in the last episode, it was clear that both Euijeong and Junmo had a soft-corner for Gicheul, proven by the fact that Junmo let him go and didn’t arrest him and Euijeong constantly telling him to stop doing bad things.
Then why did Junmo had to shoot him? I get that he was trying to stop Gicheul from killing himself so that Euijeong gets saved from the guilt of being the reason behind his death, but couldn’t he just shoot him in the leg or something to prevent his suicide? I don’t know, maybe I’m dumb but couldn’t it be simpler?
One of the reasons behind it might be that although Junmo liked Gicheul, he was blinded by the fact that he was his wife’s past-lover who was still in love with her, and Euijeong also obviously had feelings for him. So in the conflict of emotions, he might’ve decided to kill him.
The ending was very bittersweet, but all the characters received their closure so I was pretty satisfied.

I think the general idea of this drama is that the worst form of evil is deceiving somebody. As everybody in the story had deceived somebody in their life, nobody really got their happy ending. The story was well-crafted to show this.

Cast & Acting: One of the sole reasons behind starting this drama was Wi Ha-joon for me, so that pretty much tells you how well the actors did their job. This was the first time I saw Ji Changwook act, and man did he eat the role up. He was so good at it. Casting didn’t have any issues, all the actors did a great job. Bibi? Man how can she act so well as somebody who isn’t originally an actor? It’s a boon to be so versatile, really. She was awesome.

Soundtrack: It’s a webseries, a thriller one on top of that, so it doesn’t really have your typical kdrama OSTs. But the soundtrack was good, did the job for making the scenes more effective and creating an overall impact.

So, that’s about it. That is what I felt about this drama. It was a great watch for me, and I would recommend it to everyone looking for a good traumatizing thriller.