Both Pon and her sister didn’t really need to die but I guess the author must be bored with their design so they just decide to kill them off for the shock value I supposed.
This chapter alone prove why this manga deserve its rating.
Welp, the suicide is really unnecessary. I really want the lil sis to became an assassin as well... But oh well, her character design is really not that appealing so I get the author in that regard.