Responses
I mean, the story began with killing bullies, but his last gig was 'let's film how they manipulate answers for an exam' ...
Just imagine how long this story could go on, protecting Haruki from everyone in college, and when Haruki become a salary man Shirayuki enters the same company and kills everyone there, too? No, probably not

I honestly don't believe that Shirayuki will kill his little brother, but he sure will warn him not to get close to or threaten Haruki again. I guess either Nanami will stop trying to hurt Haruki because he will fear losing his big bro's affection, or plot twist Nanami will too become obsessed with Haruki and it will be a short love story because obviously Shirayuki won't allow someone else to want his beloved Haruki xD