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.

Well, if he shoot Moriaty, then the chance of him dying was 100%. If he shoot the explosives, even if there was only a tiny chance of them exploding, it was more than 0%, and so a best option than shooting at Moriaty. But I also think that maybe, if the explosives worked, all them would die, including the snipers. So, aiming at the explosives he was also treatening them. But the explanation above of the confusion of the explosion actually saving them was better.

For one, Sherlock could likely tell the bomb vest would explode. Why wouldn't they, when the same bombs were put on the other victims there wasn't a trigger, there were snipers. A gunshot is nearly sure to set it off.

His options were shoot Moriarty, resulting in also his and/or John's death as well. Or, he could shoot and detonate the bomb, killing not only the three of them, but also the dangerous snipers. Those snipers also could have managed to shoot Sherlock before he could shoot Moriarty. He had John's consent, and the bomb was the best option. If nothing else, it was a powerful threat.