It's all Mariko's own judgement. She know he's a stalker but at the same time, she also knows he's a good person who kept protecting her for a long time. Sota was on the verge of suicide and that's why Mariko told him that she won't forgive him if he disappears. He didn't set these things up and was surprised. Episode 8 will give you more answers.
Actually, there's a "knife scene" at the very beginning of episode 1 (the opening scene of the drama).…
I already guessed that Sakura is the stalker of Mariko's mom but the theory of him being the killer of Mariko's mom... I haven't think of that. We got to know Hatano's main target is Sota so about the scene in Episode 1, Hatano used Mariko to lure Sota out and that's why he came with the knife. My guess right now is Sota wouldn't kill anyone and the blood scene is another trick to deceive us.
I'm curious about how they'll find out about the detective evil deeds.
I like that the tension from the first episode still active and the suspense is interesting.