Compared to the vast majority of all other mass-murderers (who use guns), this is the first guy to shoot like a hunter. Most gun-toting killers walk into a place and just start shooting in plain sight, again and again till they run out of ammo, victims, or get killed themselves. They stay in one spot drawing attention to themselves.
This guy uses surprise and concealment, fires one shot and runs. He knows that a second shot will greatly increase his chances of being spotted.
For mass murderers, the DC area killer behaves more like a sniper than anyone ever has before. His skill may not be all that rare or unusual, but his tactics are (for a mass murderer).
eskimo