"When, by the dim and yellow light of the moon, as it forced its way through the window shutters, I beheld the wretch - the miserable monster which I had created. His jaws opened, and he muttered some inarticulate sounds, while a grin wrinkled his cheeks. He might have spoken, but I did not hear; one hand was stretched out, seemingly to detain me, but I escaped and rushed downstairs." Mary Shelley's Frankenstein Pg. 43 .
This was the monster's first experience with another life form, his creator, the first emotion he experienced was rejection. The monster seemed to be smiling at Victor Frankenstein, and that smile was returned by Victor fleeing in terror. From this moment on the monster was forced to fend for himself, being forced to learn everything in life by himself. I feel this passage shows the monster was the real victim in this story.
The monster was now forced to live out in the forest by himself, no one to talk to, no one to learn from, in extreme heat and extreme cold. He learns what to eat and drink, and how to make fire, but he still has that sense of his creator rejecting him in the back of his mind.
When the monster finds the little hut in the country where the De Lacey family lives, he finally is allowed to learn all the essential social aspects of life. While watching the De Lacey family from his little hole, he learns how to talk and form conversations, how to read, history, and geography. But, because he was not taught about social situations by his creator he is falsely sucked into the thoughts that this family will understand and accept him.
When the monster gets enough courage to talk to De Lacey, he is allowed to finally feel companionship, but it is quickly torn away from him when the children come in. After being so horribly rejected by the De Lacy family he vows to take revenge on his creator, Victor Frankenstein, by killing everyone close to him. I wouldn't exactly say the monster was justified by taking such drastic actions, but again you have to think back to when he was first created and made his first contact with humans.