Einstein seems to be trying to address the conflicts between a mechanistic view of the universe and humanity, and the desire for free will. Whether we believe in spiritual predestination, or that humans are simply bags of chemicals reacting to stimuli with no more actual independence than an amoeba, we are forced act as though people have free will to act in our own, in order to require responsible behavior.

Or, put another way, no matter how one can justify that a murderer is not responsible for their acts — whether from upbringing or because the universe is on a set course and his mechanical destiny is to murder, society still has to act as though they had a choice in the matter and so hold them responsible; otherwise, murderers would go free.