Why doesnt Victor tell anyone (his family or the police) about his creation?

is so
horrified byhe has created, which looks to him like a mummified monster, that he doesn't want
anyone else to know about it. When Clerval asks him why he seems so thin and sick, Victor
responds: Do not ask me." It seems he is too ashamed and appalled at what he has done to
reveal it even to a close friend.

Later, he gives a fuller explanation of why
he won't tell his father about the creature:

I had a
persuasion that I should be supposed mad, and this in itself would for ever have chained my
tongue. But, besides, I could not bring myself to disclose a secret which would fill my hearer
with consternation and make fear and unnatural horror the inmates of his breast. I checked,
therefore, my impatient thirst for sympathy and was silent when I would have given the world to
have confided the fatal secret.

As Victor states above,
he fears people will think he is crazy if he reveals the existence of such a monster. The
creature is not on hand to prove that Victor is telling the truth, and he fears people would not
believe he actually made life from dead body parts. Second, as far as family is concerned, he
doesn't want the people he loves to be subjected to the fear and horror that he himself
constantly experiences. He doesn't see how any could come from telling of what he has
done.

He decides to deal with killing the creature himself. Although he
doesn't this directly, we can assume he doesn't want the police to think he is
insane.