Imagine filming it..
Moriarty straps John to a bomb and we’re like: damn. you better not hurt John.
Magnussen flicks John’s face and we’re like: HOWDAREYOUYOUEVILLESTOFEVILMEN oh my god you deserve the title Sherlock gave you. You are the Umbridge of Sherlock someone throw a chair at him where is the lady when we need her to put bullets into his chest?!
I think, more than anything else, it’s about disrespect. To have John Watson, Fifth Northumberland Fusiliers, have to sit there and take something so degrading as a schoolyard taunt is awful to witness. Being strapped to a bomb by Moriarty was a thumb in the eye to Sherlock but emphasized John’s importance. Magnussen is proving right here that John is absolutely without importance and it’s heartbreaking to watch.