I probably should have chimed in with this sooner, but allow me to share my story. If it sounds familiar, it should. I've written about it here many times as it has been a large and never-ceasing issue in my life.
I'm one of three children. Both of my parents are fairly normal. Neither have any major issues. No mental illness, no addiction problems, no violent tendencies. Yet somehow, their first-born child has severe bipolar disorder, and a host of other problems that go along with it. Even though she is more than 30 years of age, she is like a five year-old in many ways. She can take care of herself most (certainly not all) of the time, but her life is VERY turbulent. She is in frequent contact with my parents and a constant worry to them (though we have all gotten better at dealing with her without getting too stressed out). My parents help her a lot, and so do her friends (what are left of them), but even with all the help, she has a lot of problems. She has had to live with my parents for extended periods, needed them to bail her out financially, needed them to take care of her during one of her quarterly episodes of severe mania, etc. I'm not sure people who haven't experienced mental illness can appreciate how burdensome it is to have a mentally ill family member to help and take care of.
That's my sister. My brother is on a whole 'nother level. He's a natural born psychopath who is no longer capable of functioning in society and belongs in a mental institution. Instead he is in and out of jail, so much so that he has spent roughly half of his adult life in jail, and the other half staying with my mom since he cannot support himself in any way and no one can stand to be around him for any significant period of time. He has destroyed his brain through long-term substance abuse (mostly alcohol and marijuana, but also a slew of other drugs). He's so messed up now that even a cup of coffee sends him over the edge. He'll start ranting and shouting in the middle of the night waking everyone up and can't be settled. He has been a huge problem his entire life, getting in fights and trouble with the law throughout childhood, constantly looking for ways to upset and harm others, even if it meant hurting himself more in the process, etc. It destroyed our family. My relatives want nothing to do with him, nor does my father, nor do I. My dad basically divorced my mom over it since she refused to break ties with him. He threatened to kill me on more than one occasion, which was pretty troubling given I was a teenager at the time and he is 5 years older than me. My parents allowed this to continue despite numerous please from me to kick him out so that I may have somewhat of a normal life, but they never did, and this created a lot of problems for my relationship with and feelings towards my parents. It basically lead to me nearly cutting them out of my life for several years and never really loving them the same as I did as a child, though I feel my relationship with both is good now.
Despite having two literally crazy siblings, I am normal, or at least not mentally ill to the point of needing treatment. My siblings, though both full grown adults, are more burden than joy, and since they are not my children, not mine to look after, I moved 1000 miles away to get away from the whole cloud of problems they produce. Because my brother has always been purely evil, I have no positive emotions towards him. I do not like him. I do not love him. I am sorry that he exists, both for the world and for himself. I want nothing to do with him. I want to hear nothing of him. My sister, though a huge burden, is still a wonderful person and has always been, so I still contact her and love her and try to help her, but to live with her or near her would be enough to pull me under. I would drown, and she would only be slightly better off. I cannot justify this, so I help her in small ways from afar.
I look at my parents and how life has turned out for them, and I feel very sorry for them. It's a tragedy. Nobody deserves this. I would not wish their situation on anyone. They will never know the satisfaction of raising successful children (outside of me) or see any promise in the future of their genes (through grandchildren). Being involved in all this but also observing everything has made me think long and hard about children... it is something I wrestle with nearly every day because I would deeply like to have children of my own, but I don't know if I could handle having a child like either of my siblings (who I share my genes with), and even if I am, I am certain my life would be better off with none than a crazy child who is a burden until the day I die. I still have about a decade to consider my options, but I really think my only hope is for technology to advance enough so that doctors/scientists can isolate "bad" genes and select against them when creating an embryo. Short of that (which likely will not happen any time soon), I really doubt I will reproduce. It just is not worth the risk. It is a real tragedy too, because I am a tremendous outlier in a lot of positive ways and feel like I have a lot to offer a child, but the tiny number of toxic traits buried in my DNA are enough to ruin all the positive ones.
I share this not really to justify not having kids but to provide an example to consider for those of you who cannot understand why some people think twice about the their own procreation. I hope you got something out of it.