There are many reasons why Victor became so fixated in the idea of building a living thing. First, he had been studying for a while the concepts of alchemy, science, biology, and most importantly, electricity. His fixation with electricity was particularly important in that Victor thought that he had conceived a notion: That electricity, that invisible and yet destructive force, can also be the link to reviving things and bring them back to life.
Therefore he embarked on a crusade to proof his theory. Do not forget also that Victor became kind of arrogant and ambitious, obsessive, and too full of himself. He was playing God and he knew it. Hence, out of a pile of dead body parts he put together this creature to which he would apply a treatment of electricity, as he thought. And therefore, not only would he proof himself correctly, but he would have achieved the amazing deed of creating life and be godlike.
In all, it was a combination of arrogance, obsession, passion for science, curiosity, and the ultimate achievement of creating life.