The creature responds to his surroundings, and Victor is largely responsible for the things that occur where the creature is concerned. Think of this: a baby is fed peas and does not like them. He throws them back at his mother and covers the room in green slime. Is he a "bad" baby for not liking peas? No, and he's not bad for throwing them, either. It's an instinct...a reaction to distaste, but it is not a premeditated action...at least not the first time.
The creature is the same. When he is created, he smiles at Victor. He even tells us in his own chapters that he was benevolent and kind and full of love for his "father" and humankind. Victor abandons the creature and leaves him to the harsh world of judgemental humans. The creature is abnormally tall, ugly, and strong. People predetermine that he is something to be feared and struck down...rather like the pitchfork scene in Shrek.
The creature is beaten, chased, and when the De Lacey family leaves him, he is without all hope of ever being accepted into society. He therefore decides to seek revenge on his creator, Victor, who has carelessly thrown the creature to the destiny that fate holds for him.
The creature explains that he just wants to be happy. When Victor again denies him this by refusing the female, the creature swears that Victor will not be happy either. I don't think this makes him evil...just human.