After watching for the first time today, I went rushing off to find a forum so that I could join the 'two poison pills, antidote in his pocket' crowd. And I like the theory about the game itself being only for Sherlock, not the others--that makes sense to me.  
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To add an additional layer of thought--I imagine that the killer murdered not only to confirm his own intelligence in a concrete way, but also in order to prepare himself for death by watching the life fade from the eyes of others in the way that he himself planned to die. I believe that any surprise registering on the killer's face when Sherlock picked up the bottle seemed to be mixed with disillusionment, and the reason for those emotions was the adrenaline-fueled knowledge that (a) Sherlock hadn't figured out his real game, leaving him victorious; having proven his intellectual capability, and (b) that means no one can beat him, which means that it's time to die. If the best out there still underestimated him, still couldn't get it, then what's the point in going on playing the game? So I don't believe he intended to take the antidote this time. I think he intended to die with Sherlock, and let Sherlock find out that way. To look into his dying eyes and have that satisfaction of the poor cabbie beating the great genius.  
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I think Watson most definitely did save Sherlock's life in shooting the killer. I think the real trick to the cabbie's 'choice between life and death' game was this: choose the pills, you die. Choose the gun, you live. Sherlock almost won, because he figured out that the gun was a bluff... but perhaps a little hubris crept in, which the cabbie seemed eager to stimulate.  Disappointing and thrilling at the same time, to know that your desired punishment will elude you; bittersweet to find that your mental script for suicide and final relief has not been followed... and to know at the same time that you've proven yourself worthy by defeating a great genius.   
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This proves