Why?

So on Saturday I'm walking down Kilburn High Road after looking inside a Blockbuster, and suddenly I hear a woman screaming that she's being robbed - out comes a teenager with several DVDs running for it. I run after him, but since he's younger and lighter than I am there's not much chance of my catching him - especially since he says "I got a knife" just before chucking the DVDs over a brick wall.
Yes, I did see a police car coming along; yes, someone asked me if I managed to catch him; and yes, I did look into looking for them on the other side of that wall (I couldn't get to them). But I should have caught the guy. I should have gone back there a little sooner. And I shouldn't have been put off by his threat.
I don't know how to feel. Proud that I tried to do something good? Bad that I confirmed what I knew all along - that I'm a coward? I'll settle for frustrated. Frustrated at myself for not getting the kid; frustrated that people will do stuff like that for a damn DVD.
And sorry that I couldn't do anything for the woman in the store; she was crying about what had happened, distraught. That was worst of all. For her sake, I hope they catch the little shit. Or if they don't, I hope the DVDs he stole freeze up.