Victor is conveying a perspective of a person who feels guilty because of what happened. He feels that he could have acted to prevent Justine's execution, but he abstained from any action, partly because he thought nobody would believe him, and partly out of cowardice. So, now that everything is over and another innocent person has died because of his creation, he feels the weight of his own impotence. He didn't act when action was possible. Now, any action would be futile and wouldn't be able to undo what was done.
Hence the phrase "the dead calmness of inaction and certainty" - his grave mistake has now produced monstrous results, and for the second time. It is not just a possibility that something bad might spring from his actions. It is a certainty, and he must live with it. He is "deprived both of hope and fear" - hope that nothing bad would happen and fear of something bad that would render him active. From the very moment of creature's creation, Victor is a passive observer much more than an active person who would claim responsibility for his actions.
