I don't think it's cruel to bring a child into the world. I agree that not all parents are made to be parents. I also agree that some parents are unfit and should not have the right to raise children. I do strongly believe that all family have ups and downs, but that most are good.
It's sad that you are feeling this way, I'm guessing you are very disappointed about you're parents. I hope you are not being to hard on yourself. I hope you have some help, someone to talk to about this, a therapist, a doctor. Take care.