Moral culpability is defined as the state of mind of a person before and while committing an act that may be considered cruel, illegal, "evil," or deviant. For example, if someone does harm upon someone else on purpose and with the intention of causing harm and pain, then this person is morally culpable for the actions he or she is committing.
In Frankenstein, both Frankenstein and his monster are equally responsible for the situations that take place.
Victor creates the monster out of pure, shallow ambition, just for the sake of attempting to be godlike.
I had worked hard for nearly two years, for the sole purpose of infusing life into an inanimate body. . . I had desired it with an ardour that far exceeded moderation; but now that I had finished, the beauty of the dream vanished, and breathless horror and disgust filled my heart.
Insubstantial and vain, Victor now rejects the very creation he fought so hard to make. He is morally culpable of doing everything, from the desecration of corpses, to the unethical attempt to mock divine intervention, just to bring life into this body. Any consequence that arises as a result of such actions stem from an act that was, essentially, immoral. For this reason, Victor is to blame. It is no surprise his creature rebels against him.
The creature is shunned by Victor, which causes its hatred. The creature demands that Victor creates a partner for him, thus allowing Victor to fix his wrong. When Frankenstein changes his mind after having originally agreed to do it, the monster punishes him. The anger and want for revenge is what makes the creature morally culpable for his actions. Even though the creature may have had a degree of reason for hating his maker, the fact that he acted against him on purpose is what would hold the creature morally responsible for its actions.