Nietzsche and Mercy/Power

There was one thing that bothered me when reading Nietzsche's second essay concerning guilt and bad conscience. In the essay, he talks about how mercy is the privilege of the most powerful or wealthy man, and in turn only the privileged are shown mercy. This fundamentally makes no sense to me. It seems that those who deserve mercy the most are the ones that have less. For instance, if in a city that is less powerful and wealthy, and a homeless man steals bread for his starving children, should he not be shown more mercy because of his situation? I see more of a need for mercy with a man such as this than a man who is in a powerful and wealthy society and who has the means to buy his own bread, but still steals such. Furthermore, is it actually a question of wealth at all at this point? I am a little confused as to what exactly he means by “powerful.” What marks a powerful society? Is it wealth, or something else, especially when thinking about his definition of good and evil in the first essay? How then is power defined?