No, I think you are wrong. The police say (and I quite well believe) the victim(s) were targeted. The shooter clearly must have expected to be in a firefight with the metal detector Capitol Police who man the metal detector machine, but otherwise, this was a target-enriched zone at that moment. He shot at no bystanders, apparently.
From what I gathered, the perp was a convicted felon just released last year from prison. How did he get a gun?