The idea of the "pond scum" and broken people making each other worse out of fascination and a sadistic sense of pleasure is something that Shuzo Oshimi's likes to carry very heavily. In one of his works "Aku no hana" I remember this exact phrase being repeated throughout. I think the author likely had to go through a similar realisation around the same age as Isao or while growing up since it seems to be such a personal theme.