Well, there were heaps of reasons for that. There are always reasons for murdering a man. On the contrary, it is impossible to justify his living (5.16).
This final line gets at a fundamental problem which existentialism tries to solve: if the world is indifferent and there is no grand "purpose," how do we justify living at all? How do we give our lives meaning? Philosophers like Jean-Paul Sartre - contemporary to Camus, and rival when The Fall was written - would answer that a man is defined at any moment by choosing to act. Does Jean-Baptiste exemplify this "solution"?