This chapter is very cathartic. I'm sure a lot of us have imagined at least once in their life when it comes to emotional abuse. While the ritual Seiichi did with Fukiishi was therapeutic, the shadow his mother cast is always with him and he still harbors questions and rage towards her. What this chapter is trying to convey is that while he is pointing the finger towards her and wants her to disappear from his life, he still holds a lot of guilt. You can't ask her to disappear and feel guilty at the same time.
His mother is really good at imprinting that guilt on him and uses that to control him. The phrase where he says, "Always, without saying a word...you'd stare at me like you had something to say...!" is a form of guilt tripping someone. It's a way to judge someone without actually saying anything and that gaze is fucking agonizing. The more she stares the more guilt he feels. His mother feels no guilt which is why she's "clean" while he still "filthy" with his guilt.