Why did Holmes aim at the explosive rather than at Moriarty?

If Sherlock shot Moriarty, then the snipers would kill him and Watson, end of story.

If he shoots the bomb, there's a chance they all die, but there's also a chance to create enough of a mess to be able to escape. If the bomb exploded, the snipers wouldn't try to shoot, since they'd risk harming Moriarty.

So detonating the bomb was the best option.

To expand about the uncertainty of the situation, it's the best option for Sherlock. Remember that him and Moriarty are the kind of people that can think pretty much every outcome of a situation. Sherlock probably realized that he has no option to escape in the current situation, so creating a situation where neither him nor Moriarty can predict the outcome is the best (and probably only) solution for him.