What Shiikawa did was petty and immature but do not denounce him as a character without fully understanding his reasoning. I will preface that I do not think his reasoning justify his actions. But I can understand his disdain for Satoko, as her relationship with the boy is something that he wanted. Not necessarily a 1:1, but a relationship where both partners are equally involved and loved. Other than this instance of immaturity we have nothing else that pushed the assumption of him being a bad guy, and the flashbacks briefly show that he is a caring person. If not shown in there talk.
Just as how Satoko was a victim of Shiikawa, Shiikawa was a victim of Satoko. An eye for an eye and although as I stated his actions arent justified. At the end of the day all he wanted was his relationship with Satoko to work. Over all hes a good guy so I do empathize with the character. I mean you could say that both of them suffered from the others immaturity.
