Notes

in ASiP, John foils the cabbie. Both are short, light-haired, unassuming men in jumpers who are usually overlooked and underestimated (by everyone, though in this case by Sherlock especially). Even the pre-series events sort of parallel each other: the cabbie was diagnosed with an uncurable condition and could die at any moment, and John almost did die and now suffers from PTSD and nightmares. They both live with death constantly looming over them. Also, both are approached and offered money to do less-than-morally-acceptable things by wealthy, dangerous men. The cabbie is hired by Moriarty to kill and John is approached by Mycroft to spy on Sherlock; Sherlock even calls Mycroft "the most dangerous man [John] will ever meet", which is a title most would afford to Moriarty. It foreshadows Moriarty's danger and villain status later on in the show (and foils Mycroft with Moriarty, but that's a whole 'nother thing). And this might be a personal interpretation, but judging by Mycroft's assessment of John ("you're not haunted by the war; you miss it") and John's obvious depression and love of danger, I believe the cabbie might have been set up as a (rather extreme) example of what could have happened to John had he not met Sherlock. People in the "Sherlock" universe always warn John away from Sherlock but by the end of the first episode, John's straight up murdered someone. True, it was to save Sherlock's life, but the fact that he was so blase and unaffected by killing despite his nightmares and the fact that he's an obvious adrenalin junkie <em>and</em> his knowledge of the human body all point towards a scarily capable serial killer, had John gone down that path. The cabbie did, and I think it's only thanks to Sherlock that John didn't do something just as drastic