The creature is angry only after he realizes the method in which he was created, the abandonment of his "father", and the fact that he was not only abandoned, but created to be extremely ugly so that any hope for inclusion in human society is obliterated.  Remember that the creature has been witnessing the loving behavior on display in the de Lacey family--the sacrifice made for loved ones, the caring for someone who is unable to see and care properly for himself, the total acceptance of the foreigner.  The creature hopes for this same love and acceptance, and it greeted with screams, fainting, and sticks on his back.  It is only at this point that he feels rage and vengence boil in his blood which he channels at his creator and is temporarily satisfied by killing little William.  The creature was good, kind, and benevolent before he suffered complete and consistent rejection...even with William, he hoped to train the boy to love him.  It is only with the discovery that the boy belongs to Victor's family that he crushes the child's throat.  Once this has been done, he is fed with the feeling of satisfaction that comes from getting even with Victor.  It continues with the framing of Justine and the murders of other family and friends who have Victor's love.  The creature, had he received love and acceptance from Victor and other members of society, would never had acted in such a violent and evil way.  He acts with malice and premeditation only to seek vengence on the father (who also acts selfishly and perhaps with evil in his heart when he runs away from the child he created and leaves it to fend for itself) who abandons him.

Where is the evil in the thoughts and actions of the monster? Do you think that he and Victor each represent one side of human nature?

It is hard to say that the monster represents one side of human nature, because the monster is a product of his surroundings and of the treatment from his creator, Victor Frankenstein. The monster is at first very nurturing and caring. He wants to belong and tries to take care of the Delacy family. This is even after the fact that Victor has been repulsed by him and alienates him. The monster only seeks acceptance. He wants victor to create a mate for him and promises to leave the world of normal man. Victor assumes responsibility for eradicating what he perceives as a mistake when really he should assume responsibility for teaching and training his "son" to fit into normal society.

The other side of human nature is seen when the monster is refused a mate, cast out of his "family" and treated poorly by virtually all normal men, save old man Delacy. True, he should not have murdered Victor's little brother, but the dark side of human vengeance is irrational and impulsive. The monster wanted to take away something that mattered to Victor, as much as he lost something at being alienated.