In Frankenstein, the monster is originally good-hearted, wanting to be accepted by the people around him. After many negative experiences with society and his creator, Victor Frankenstein, his monstrous acts become an obvious future. The monster says it best himself when speaking to Victor, stating "I am malicious because I am miserable. Am I not shunned and hated by all mankind?" (Ch. 17)
Notably, after his creation is successful, Victor immediately exits the room, incapable of looking at and accepting the abomination he has created. He wishes he could reverse his unfortunate decision. This rejection is replicated throughout the story as the monster interacts with different townsfolk. After witnessing how they communicate and love one another, the monster seeps deeper into isolation, attempting to find his place in the world. Further interaction results in further rejection that increases in severity, leaving the monster bruised, beaten, and even shot when attempting to save a drowning villager. These experiences drive the monster from seeking acceptance to seeking revenge, especially against Victor Frankenstein for bringing him into existence in the first place.
Society and Victor's rejection of the monster due to his appearance had isolated him against the world, persuading the fiend to abandon his hopes of acceptance and commit these monstrous acts of murder, assault, and revenge.