Misalignment of rewards, the root of all evil

My day in London ended in a subway station (“the tube”). A woman approached me asking for change; I tried my best to help. I had 4£ ($6) in my pocket that I pulled out when I realized she was really a beggar. Yet I was not fazed. I had retracted some pounds from my tip at dinner because the Indian buffet had a completely ludicrous policy against changing plates between courses and thought the change would be better suited in the coffers of a mendicant. How wrong I was. After giving her all my change, she asked me for more. I explained to her that I only had bills to which she said she could give me change. She pulled out a handful of random sized coins that I am unfamiliar with, but probably added up to not much. It was clear my 4£ donation was already the most generous of the night. But as with a PhD in organizational behavior she knew that her foot was in the door and I might be converted to donate more. But as fun as learning the British coinage system sounded, I am far too much of a millennial for a pocketful of change. I politely declined. She gave me a rather disappointed and slightly malicious look as she was leaving, proceeding to pester other tubers. The next  victim simply said no and was left in peace. Somehow, I felt bad. I had done a good thing and yet I am sure I felt worse than my successor who simply said no. If justice prevailed perfectly, there would be no evil. But because sometimes doing a good thing leads to disappointing results, that good is often never done.