"From that moment I declared everlasting war against the species, and, more than all, against him who had formed me, and sent me forth to this insupportable misery." (page 97)
At this time, the novel begins to take on a much more sadistic and malicious tone than it did before. Neglecting his melancholy tale, the monster begins to explain to Victor why he changed from a good-natured monster to a bad one. Stemming from nature v. nurture, it could easily be argued that the creation became the way he was due to other events around him. Because he was neglected and left with no companion, he was driven to look for some sort of friend elsewhere. Not stopping to think through the consequences, he decided to wage war against mankind for his frustrations and absolute misery. Although he was a kind being by nature, no one ever got to know him long enough to realize that because they were so hideously disgusted by his face and his terrifying nature. It just goes to show you how influential one's appearance can be.
Oddly enough, the creation didn't seem to think that Frankenstein's companionship would be enough. Instead, he used Frankenstein's brilliance as a weapon of manipulation : he asked for a lady creation in return for his disappearance.