I kind of skimmed over a lot of details just to cover the twist. The reveal itself is pretty random but theres also an explanation for why the MC is acting like hes just a bystander. The two people he hired were actually a pretty normal couple but heavily in debt and the MC brainwashes them into becoming human trash and gave them directions to ruin his family (2nd year psychology major btw). This goes on for 3 months before he checks on them.
So when he does arrive everything he is seeing is basically a surprise to him. But honestly its not really good writing, they give us too much monologuing from the MC for it be believable, they should've kept a lot of it out. But I guess hes also a psycopath so no point in trying to reason it out. Anyway its really just the first 6 out of 36 current chapters, its the premise not the actual plot of the manga which is a guy who kills everyone around him to get with his sister. Like those mangas where the MC kills someone in the first few chapters and spends the rest of the manga getting caught or maybe getting away with it.