You're touching on an important and somewhat confusing subject matter. I think most of us would agree that parents who tell their kids that they're "losers, disgusting, and will never amount to anything" are probably sucking the spirit, joy and goodness out of their children (not necessarily, but there's a good chance). On the other hand, I think most of us would agree that parents who tell their kids that "they can do no wrong, that they're superior to other people and that most other people are stupid, incompetent and beneath you" are probably creating a monster. So, the difficulty lies in figuring out how to let your children know (or giving them experiences which will help them learn) that they're loved unconditionally, that they're worthwhile human beings, that they should be thankful for what they have, that they will need to work hard to accomplish their goals and that they have every right to feel confident about themselves.
Personally, I have struggled a little to figure out how to be a good parent. I worry that I may have done things to negatively influence my kids at times. And, let's not forget there are probably plenty of instances where parents do a pretty decent job, but things don't turn out the best, or where parents were abusive and the children turn out to be great people. I imagine there are people whose "wiring" is different to begin with due to genetic misfires, stressors during pregnancy or infancy or physical problems at birth that end up affecting personality.