The repeated rejections by society contaminate the heart of the Creature, and thereby make him become an evil creature, full of the wicked ways of society.
First, the creature is rejected by his own creator, Victor Frankenstein; however, the creature does not recognize this abandonment as such until he learns to read.
Second, upon encounters of villagers, he is repeatedly attacked and shunned because of his appearance.
Third, after the creature learns to read and reason, and he learns of his horrible birth by Frankenstein, he attempts to befriend the blind old man, De Lacey, and again is shunned and attacked, and the De Laceys abandon their home in fear of seeing the Creature again.
When the Creature realizes that the De Laceys have gone, he vows eternal revenge on mankind, but more so on Frankenstein.
Thus, it is the repeated ostracizing of the Creature by society that contaminates the heart of the Creature and turns him into a sanguinary monster.