People are of course taught morality and ethics while growing up. Even those with a horrible childhood are taught horrible ethics, often by example.
Each person can choose whether to accept or reject their upbringing.
A very large number of those with horrible childhoods become good people as adults.
A much smaller, thankfully, number of those with good upbringings choose to behave monstrously.
I'm listening to a book right now on the Leopold and Loeb case. Both had a perfectly normal, indeed highly privileged, upbringing. They chose to turn to evil, out of boredom and a highly mistaken superiority complex.
On re-reading my comments above, I realize I've been too dogmatic. Some people really are insane, others choose to do evil for a variety of reasons, and some are perhaps a combination. Very hard to tell which.
What is “THO” ?