I have run across people who feel as you do.

I dated a girl for a long time who had her tubes tied so that is was impossible for her to have kids because she refused to take the chance of passing on her depression. She had depression pretty bad.

I had a daughter because we both wanted kids. I can see my genetic traits in her. Alcoholism/addiction and depression and anxiety. I think all my recovery and hard work has made me a much better parent as Pegasus has said. She had struggles throughout all her teen years and even an attempted suicide. Today she is 21 and is the most happy person you could meet and is getting a 3.5 in college. I still worry about her very much but she knows how to get help and has lots of supportive family. We each have our own journey and if you believe in the evolution of spirit and that we live on then I don't see a problem bringing a child into the world. If you believe this life is all there is and it will be one of constant struggle than I see your point.