Obviously you guys haven't been robbed before. It's a scary experience that can lead the most levelheaded people to otherwise erratic behaviour. Admittedly, they were only teenagers, but he's a senior (63 year-old) facing four young men. It's not hard to imagine how easy it would be for them to overpower him.
As well, you have to remember that this wasn't premeditated. He didn't start the encounter. He reacted out of instinct and I think the jury was right in acquitting him.