There is an epilogue to the story: after the incident I crawled to the phone and called 911. The police arrived a few minutes later, and a few minutes after that told me that they had caught the perpetrator. The put me in the back of a police car and drove me out to where they had a hispanic kid, maybe 15 years old, sitting on the curb in handcuffs. They asked me if I could identify him and I said no, it was dark, I couldn't see anything but a muzzle flash. So they let him go. I have no idea if I let an armed robber back out on to the street, but I've never lost a minute of sleep over that decision.
You own your actions, not your feelings.