I’ll answer as best I can, but this is only an educated guess. The amount of possibilities and outcomes that can happen during the creation of a human are pretty much impossible to quantify.
I’m sure you know it’s not “versus”, but combinations. The brain’s physiology is so fragile it’s scary. Not much is known about the formation of the body and brain during embryology, but it is definitely an imperfect process: imagine all of those billions of neurons and trillions of synapses being formed in our brains. There are and always will be mistakes.
Like building a computer or something: the smallest change can make a difference of who knows what while we’re developing in the womb: what if the mother drinks or smokes? What if she has excellent nutrition? What types of genes are we looking at?
When the child is being raised is the same thing. The how, what, whens, whys, and wheres are too much too take into account, but we also have learned a lot and will continue to learn a lot. For example, when a woman smokes during pregnancy, nicotine can actually concentrate itself in the placenta. Random things like that can make a big different in a child’s development.
I don’t know what is known about serial killers and people with mental disorders versus people that don’t have them, and I imagine it’s just as difficult to really pinpoint an exact answer, but as long as we follow the basic things we learn about raising kids to adults there’s not much we can do besides that.