Can being abused by a parent during childhood cause people to grow up into Jekyll-and-Hyde monsters?

Might be a contributing factor.

As are many other potential things.

Mental comprehension of situation.
Possible behaviour disorder.
Hormone imbalance.

I'm sure it would probably contribute to personality problems. There are all sorts of factors to take into consideration - the degree of nature/nurture.

I believe so.
Some people develop different ways of coping.

Even if it was all they knew as a kid...sometime somewhere they had contact with examples and influences of good people and what is the right way to behave and live...so it all comes down to a choice as to which direction they go.

Yup