Schizophrenia is one of the disorders where one may hear voices, however, the VAST MAJORITY of schizophrenics are not violent, the disorder can be treated with medication, and there are various levels of schizophrenia ranging from mild to severe.
I absolutely can't believe what you're saying. In your opinion, a person who has a disorder, with no violent past and no marks against them other than that they are disabled, should not only avoid having children, but be prevented from doing so if they wish. Some developmentally disabled people, with very low IQ's and mentallities should not have children. That does not mean you keep them from having one. I'm sure that doesn't make sense to you.
Let me put it this way. Are you really concerned for the child, or are you more concerned for yourself? The way I see it, most people are more concerned for themselves, worried about the behaviors and possible disabilities of these children as well. What does that make you? A genetic supremecist, a person who, more or less, believes that people with certain disabilities, disorders, and behavioral traits should not be allowed to exist. If you can't kill these people outright, you'll just keep them from having children to prevent the perpetuation of the trait.
In fact, while we're at it, how about we keep people with Alzheimer's from having kids. And, you know, ADD is really annoying too. You can forget about OCD, those people sure shouldn't have kids. Genetic Cancer trait? They might die too soon, better keep them from having kids, and their kids might get that trait too, better that they never exist in the first place and drag down our medical system. You can't have a mom who has seizures, so people with epilepsy are out. The mild schizophrenic who's in treatment? Well, they haven't displayed any violent tendencies, but they could get worse, so they're out. What about bi-polar disorder? Might be kind of wacky growing up with that, so they're out. And hey, you know what? Certain ethnicities seem to exibit more mental disorders than others, so we'll keep all of them from having kids too.
You see where this is going?