The monster refers first to Sorrows of Werter. He makes several observations of the text and its characters that could indeed have as easily been used to describe the monster's character: "The gentle and domestic manners it described, combined with lofty sentiments and feelings..." As the monster's story unfolds, we see in his observations of his "host" family that he inserts himself into the domesticity of the situation, both literally and figuratively. He does chores around the property to aid in the daily lives of the family; in turn, he projects himself into their quarters through his "Peeping Tom" hole. Although not as easy to rationalize for his overall destructiveness, there is a gentleness to his character: "I longed to discover the motives and feelings of these lovely creatures; I was inquisitive to know why Felix appeared so miserable and Agatha so sad. I thought (foolish wretch!) that it might be in my power to restore happiness to these deserving people." This passage also illustrates some of those "lofty sentiments" from Werter.
"'The path of my departure was free,' and there was none to lament my annihilation." This concept could have been the undoing of the monster - and of Frankenstein in turn. If internalized by the monster, the realization that there was no earthly thing to hold him back justified basically every murderous, hurtful act. Yet the monster did not lack conscience: "I had been accustomed, during the night, to steal a part of their stores for my own consumption, but when I found that in doing this I inflicted pain on the cottagers, I abstained and satisfied myself with berries, nuts, and roots..." Indeed, the monster could be summed up in the keen observation of Werter: "...his character contained no pretension, but it sank deep."
The monster's interpretations of Paradise Lost also lends to the dichotomy of his character. He observes: "Like Adam, I was apparently united by no link to any other being in existence..." His hideousness and exile made him feel like an Adam, but far different than the one in Milton's classic: "He had come forth from the hands of God a perfect creature, happy and prosperous, guarded by the special care of his Creator..." The monster was "born" an atrocity and abandoned immediately by his "father", his creator. One could not help but feel more than a little rejected! As for the monster, he blamed his sorrows and woes on his creator, the same as any of us (who believe in that sort of thing) might curse God during times of strife and hardship. The monster purposefully kills those close to Frankenstein as his revenge against his creator, to hurt him. This seems a very human thing to do, as people might subconsciously hurt others by hurting themselves - through drug abuse, promiscuity, and other "sinful" acts - as a way of getting back at God.
Finally, from Lives, it is worth noting the monster's observations here. For one, he says that Plutarch "elevated me above the wretched sphere of my own reflections..." He also states: "I felt the greatest ardour for virtue... and abhorrence for vice...as I applied them, to pleasure and pain alone." As the monster equates pain with vice, he abhors the pain Frankenstein has caused to him through creation and desertion, and therefore abhors Frankenstein. And his self-described "elevation" further rationalizes his destructiveness by raising his heinous acts over those of his father. He little denies them, nor does he accept responsibility, other than to tell Frankenstein, "You made me do these things."
When I first began to write this piece, I prefaced with a paragraph about how I felt that the monster was only partly represented in the readings he mentions, that a good portion of his character is the reflection, the antithesis of Frankenstein himself. But perhaps the inverse is true. Maybe the character of Doctor Frankenstein is the antithesis, and the monster was indeed the construct of the grand concepts of Sorrows of Werter, Lives, and Paradise Lost. The truth is probably somewhere in between, as we will never know what Mary Shelley had in mind specifically, or how she arrived at such brilliant, tragic characters.