This being a Greek tragedy and all, there is lots of ambiguity about the question of whether individuals have free will, or whether they are just controlled by fate. Just as in Agamemnon, Part 1 of the Oresteia trilogy, problems of fate and free will are closely linked with the overarching themes of revenge and justice in Libation Bearers. On the most basic level (not that it's very basic), these themes are connected because the idea of fate seems to give the idea of justice a hard time. After all, if people are fated to commit crimes, how can it be just to punish them?
Based on what we see happen in Libation Bearers, it is impossible to tell whether free will exists or not.
In the world of Libation Bearers, free will is irrelevant; those who commit crimes must be punished, whether or not they intend to commit them.