Gosh. I don't even know. I was never weak to gory or psychological stuff, but this has got me bad!
Visually, the first thing you see in chapter one is good art (i.e. normal proportions, detailed clothing), nothing too AMAZING for a webtoon nowadays. But it's dark. Literally. Nothing saturated and orange-y like Cheese in the Trap. Everything is pale and gray. Again, nothing too strange for a webtoon, since a lot of them (besides romance) tend toward more wash-y colors.
Nothing is strange until...BAM! You get hit with this Levi-haircut, sleep deprived dude who looks suspicious AF. (Funny story, I was avoiding this for a while because I thought the cover was for a Levi X Erwin AOT dj.) NOW it's unique. It's not afraid to make its main character look like a loser who has been living in the sewers (which are somewhat clean) and feeding off tiny rat-skewers. He looks scrawny; his clothes are sloppy; and he looks haunted.
And it's not only that the MC is a loser, it's the fact that the artist had the confidence to hit us with a creepy and loser-ish MC that lures us in. What about this story will be addicting? Obviously not the beautiful characters. The artist has an ace up their sleeve, and we want to know what can make such a grotesque MC likeable.
Now to the actual writing. I swear. This is what got me. Same-old "I have an unrequited love, but a certain condition makes me unable to confront the person I like" hook with the narration. First twist: they stalk their love interest on the internet (like we have all done once). Second twist: the speaker is a man who likes another man. Okay, we went in knowing there was going to be some ahem stalking going on, and we'd know it was a BL if we read the tags.
But wait, the second twist hits us late, because the panels show a GIRL instead of our MC (to trick us, I suppose), but this seems almost too heavy-handed. It is, but it somehow works. We are forced to ask ourselves this, "if it's not that creepy when a girl stalks her crush on social media, why is it creepy when a guy does it?" Double standards. So, now it's not creepy when a guy stalks his crush on the computer. And then we're hit AGAIN. We learn that our MC stalks his crush in real life. Okay. This is creepy no matter who does it. I believe that this sort of thing is what gives Killing Stalking the whole "roller coaster" feel. The author misleads us a bit too obviously; they give us what we knew they were hiding; and then they hit us with another surprise that leads us to question our assumptions.