I tend to agree with Dan's point as well. The dude should have called the authorities - taking a Charles Bronson vigilante stance is stupid, and shooting a cat in a public place like that is dangerous. What if a homeless person also happened to be sleeping under the bridge when he tried to shoot the cat?
Besides, how does he know that THAT particular cat was the guilty one? Was he witnessing it eat one of his beloved shore birds when he shot it? Or was it simply the first wild cat that he found in the area?
I think this guy was an asshole with at least one screw loose, and his behaviour disgusts me. He deserves at least a good ass-kicking.