How do I connect Frankenstein and Euthyphro to the question of what are our defining traits as humans?
Euthyphro meets Socrates when Euthyphro is about to prosecute his own father for murder. He insists that what he is doing is right despite the fact that the accused is his kin, because murder is murder. Socrates questions this (of course!), as he wonders why Euthyphro would be the man to stand against his own father; Euthyphro replies that right is right and wrong is wrong, regardless of the status of the accused. His stance illustrates the human belief in and desire for justice, while Socrates' misgivings about the situation illustrates the human urge to protect one's family.
Their conversation continues, and Socrates asks, in essence, "Is the pious loved by the gods because it is pious, or is it pious because it is loved by the gods?" (This is known as Euthyphro's Dilemma.) In essence, If the pious (the good, in other words) is loved by the gods because it is good, then morality exists separately from the gods and the gods can be judged by it (and they lack omnipotence, as they do not determine morality); on the other hand, if good is good because it is loved by the gods, that makes morality arbitrary, as the actions of gods are arbitrary. Socrates's question illustrates another aspect of human nature: the desire to understand the nature of divinity. At the same time, Euthyphro's eventual response--to suddenly find somewhere else he has to be,presumably because he cannot answer the question but is a pious man and does not want to question too deeply, illustrates our desire to cling to our comfortable beliefs, even if further rational thought might call them into question.
Frankenstein, on the other hand, illustrates many of the frailties of the human condition. Victor Frankenstein is seized with grief upon the passing of his mother. What isn't natural is that he decides to recreate human life from body parts he has plundered from graves. When at long last he brings his creation to life, he is horrified by "this catastrophe" he has wrought after two years of labor. He finds his creature hideous and repulsive, and runs away. This his a silly one, perhaps, but we do abhor the unsightly and fear monsters, even if we've created them. The creature experiences the same treatment at the hands of the "barbarous villagers." We fear that which is strange to us and which we do not understand, so this is natural human behavior.
The best insights arise from the monster's tale of how he made some sort of life for himself, hiding and watching an impoverished family. He witnesses, with longing, the "love and respect which the younger cottagers exhibited towards their venerable companion [the old, blind man]," desiring love and acceptance himself. He witnesses the sadness of the younger people in the cottage, and eventually surmises that this is because they are poor. Sometimes, they go without food so the old man can eat, and this kindness moves the monster deeply. This points to more defining traits of the human condition: compassion and love on the parts of the younger people and empathy on the part of Frankenstein's creature.
The creature also has a desire for language and knowledge, other inborn human traits. Unfortunately, most of what the creature experiences is rejection and loneliness, highlighting one of the most important defining human traits: that we are gregarious creatures who crave companionship. When he finally shows himself to the family he's been watching, they freak out and drive him away, leaving him despondent and enraged. Here, he turns on Frankenstein and says, "Cursed, cursed creator! Why did I live? Why, in that instant, did I not extinguish the spark of existence which you had so wantonly bestowed? I know not; despair had not yet taken possession of me; my feelings were those of rage and revenge." Sadly, when we are ostracized and hurt, we too lash out. We want others to understand our pain, so we create pain.
You can see here, also, the creature's desire to understand why he even exists, which could be said to be another defining trait of humanity, and why some would argue that we created gods (instead of vice versa)--so we would feel that we have a purpose.
At the same time, the creature has the natural affinity of (non-sociopathic) humans to protect other humans. He's learned to hide from people, but during one such episode, the following occurs:
I was scarcely hid when a young girl came running towards the spot where I was concealed, laughing, as if she ran from someone in sport. She continued her course along the precipitous sides of the river, when suddenly her foot slipped, and she fell into the rapid stream. I rushed from my hiding-place and with extreme labour, from the force of the current, saved her and dragged her to shore.
Unfortunately, the "rustic" who rushed up to him to take the girl from his arms shot him, and the creature says, "The feelings of kindness and gentleness which I had entertained but a few moments before gave place to hellish rage and gnashing of teeth. Inflamed by pain, I vowed eternal hatred and vengeance to all mankind." This is perhaps melodramatic--as this literature is wont to be--but still mirrors the sort of emotional reactions we have when we feel wronged. In Geneva, he tries to befriend a young boy, thinking the boy would not be prejudiced against him due to his youth, but the boy calls him names and threatens to get his father, M. Frankenstein, to hunt down the creature. Upon learning who he is, the creature decides to murder the boy in cold blood--for revenge. Sadly, revenge is also a defining human trait.