The quoted line comes near the end of the novel. As Victor Frankenstein admits that he is near the end of his life, he talks with Robert Walton about how he behaved, his “past conduct.”
In the sentence immediately preceding this one, he pronounced “justified” his desire that the creature should die. More fundamentally, he is referring to developing and carrying out the plan that brought the creature to life and to the subsequent actions that the creature carried out—including murder.
In saying he is not “blamable,” Victor means that he does not feel guilt or remorse for wanting to kill the creature he created; he does not at all equate his bloodlust with that of the creature. Neither does he regret engaging in the initial, bold experiment. Instead, he makes an intellectual separation between what he did and what the creature did, rationalizing that the creature had an evil nature that drove it to kill.
Victor refuses to take responsibility for the consequences of his hubris, as he played God by animating what had been an assemblage of dead body parts. Instead, he considers the creature to be responsible.