Very slightly based on the prompt above (something I've always wanted). :P
Many of Sherlock's enemies believe that they should be afraid of John, because John is a professional soldier, an excellent shooter, who was a captain (!), has absolutely no problem killing 'not very good' people (which is everybody who wants to hurt Sherlock) and never feels bad about it afterwards. He's also loyal and violently protective of Sherlock and easily killed for him the very first day they'd met. At least some of them also believe that John is Sherlock's boyfriend (who doesn't these days?).
Maybe in criminal world there are even some popular legends about John Watson that make people feel uneasy.
All of that leaves fewer people who want to hurt Sherlock or maybe want revenge for something. You don't want to become John Watson's enemy.
It would be interesting (but not necessary) to see Sherlock's reaction if he learns about it.
GEN or John/Sherlock. Sherlock's enemies know they should be afraid of John.
Many of Sherlock's enemies believe that they should be afraid of John, because John is a professional soldier, an excellent shooter, who was a captain (!), has absolutely no problem killing 'not very good' people (which is everybody who wants to hurt Sherlock) and never feels bad about it afterwards. He's also loyal and violently protective of Sherlock and easily killed for him the very first day they'd met. At least some of them also believe that John is Sherlock's boyfriend (who doesn't these days?).
Maybe in criminal world there are even some popular legends about John Watson that make people feel uneasy.
All of that leaves fewer people who want to hurt Sherlock or maybe want revenge for something. You don't want to become John Watson's enemy.
It would be interesting (but not necessary) to see Sherlock's reaction if he learns about it.