This is the answer. He who calls first gets more of a pass than he who calls last. Perps don't call in their crimes generally. Furthermore, you were the victim of an assault, which should be reported. It was not a successful assault, but an assault nonetheless. You have important information to identify the perp- distinguishing characteristics, clothing, etc. It could help prevent another incident if he's caught and ID'd (in an ideal world).b. realize someone may have heard the yelling and saw you with a gun in your hand and the guy running away, and may have called 911.
You call 911 to report the incident and the fact that you drew to protect yourself
Additionally, what if the perp did call the police? "A crazy guy with a gun tried to rob me, but I ran away fast and he didn't shoot at me! He was about 5'8', 160 pounds, caucasian, brown hair, shoulder length, with tats and a ring through his nose!" "He looked really scary!"
Welcome to DC!