The first possibility is that the banker has learned a valuable lesson about not being a huge jerk. There he was all set to kill the guy, when all along the lawyer had no interest in his money at all. So the reason he feels bad and cries is that he suddenly sees that he's been way too obsessed with money.

At the end of the story, after the banker has learned an important life lesson from the lawyer about the meaninglessness of material wealth (and seemingly grown emotionally), he immediately reverts to his morally corrupt ways an example of situational irony.