The monster Frankenstein is completely unaware of morality and doesn't understand the cultural code of behaviors for the world in which he's been created. His problem is that he's never been "taught" or raised, and that he is forced to form what would be his own rules for living by watching the examples of those around him.
It is important to that Shelley was an atheist, and that Frankenstein develops his code without any knowledge of God or creator. Her purpose in this may have been to support the secular beliefs that morality is “inborn.” On the other hand it would be impossible to ignore that the bible and religious learning influence many aspects of the story. For example, much like the story of the prodigal son in the Bible, the monster in Frankenstein is reduced to sleep with the pigs and live like an animal. What lesson is there to be learned in seeing these comparisons?