When someone reacts to an instance of bad parenting by saying that people like that shouldn't have kids, I laugh hysterically to myself. Why? Because if those parents had chosen not to have children, the same people who are criticizing their behavior would probably be giving them just as much grief about why they don't have kids. As a girl who has never wanted children, I can count on one hand the people who support my decision. And by support, I mean not initially reacting like I just told them I killed someone, and accepting that I am sure of this and not telling me that I'm too young to know. Whenever someone talks about their future assuming that they will have children, everyone accepts it. No one says they're too young to know. But if you don't want children, it's always, "Well, you may change your mind."
So when you see bad parenting and think to yourself that some people shouldn't have children, ask yourself how supportive you are of people who actually don't have kids because they don't want them. I often wonder if we would have so many abusive and neglectful parents if it were completely socially acceptable not to have children - if having children was a decision that everyone thought about very carefully, rather than assuming it's what you're going to do when you grow up.