Sorry, Asylum Seeker, I have to touch on this a bit; and a comment just couldn’t convey quite how incorrect all of you are. Developmental plasticity only goes so far to explain the spectrum of observed personalities. We have, almost certainly, a degree of influence from our genes. This comes from the fact that genes don’t just “make the body” and then “stop working.”
Allow me to explain this further: genes and environment are not separate entities, but part of the same puzzle when it comes to behaviors. Given a very specific set of parameters, each neuron will always give the same outcome (from here comes determinism). The problems arise when you look at ALL of these neurons in a gradient of hormones, signaling agents, and neurotransmitters, the gradients make neuronal firings probabilistic rather than straight forward. Another aspect which takes a corner off of determinism is that gene activation is responsive TO the environment. This allows for individual or sets of genes to be turned on or off, as well as having varying levels of expression. While the most extreme individuals require a combination of genes AND environment to give such a result, most individuals receive one or the other to varying degrees.
As for the deterministic nature of observing a single neuron, when you look at the entire context which it is located, as well as the environmental signals it has received and is currently receiving, one must look at behaviors as a probability. Hence we do not have nature vs. nurture, or genes vs. environment, but rather a complex interplay of the two which yields the behaviors of the individual.
As for the diminished empathetic response, that can be a learned behavior associated with attenuation to a given reflex. There may be a genetic component, but we cannot, currently, conclude either way.
Some genes are known to affect behaviors, but not to the extent that you will get violence or obsession, instead, you see tendencies which can result in positive feedback mechanisms: more violence leads to empathetic attenuation, which results in more violence.