There's a belief that the Kudan came because one of them was cursed. Shinji believed that to break the curse, the cursed one must be killed.
Shinji came from the hospital after slaughtering the yakuza being led to believe that the cursed one was Hikaru. But Shinji doesn't know this can't be true, since Chizuru had already attempted to murder Hikaru but failed since Hikaru too has become immortal, just like herself. (Actually, Hikaru tried killing himself to atone. But you get it.)
In case anyone forgot, young Hikaru was the kid that got into the accident that led Chizuru's taxi driver dad to suicide. This is the reason why all of sudden in this chapter, Chizuru's mom tries to kill Hikaru too because he blames him for her husband's death.
The thing is, only last chapter we discover the suicide wasn't really Hikaru's fault since from notes he left behind, it turns out Chizuru's dead dad had also been seeing the Kudan before his death.
It's recently hinted the cursed one was actually Youji(? The big guy, the one who's NOT immortal and got hospitalized) and he's the one pulling the strings.