Nature (genetics) and nurture (environment) work together to produce every phenotypic trait we exhibit, including behavioral tendencies. There is no extricating nature from nurture, or vice versa, so everything we are is a product of both. Psychopathy, for example is an inherited neurodevelopmental disorder which can become behaviorally dangerous through environmental exacerbation. On the other hand, sociopathy has a stronger environmental component than genetic, but the pre-existent genotype of the individual, when influenced by environment, produces sociopathy; which is why not everyone who has a bad childhood becomes sociopathic (because their genotypic traits were not conducive or amenable to it). That aside, both psychopathy and sociopathy indicate a diminished sense of affective empathy (as opposed to cognitive empathy). Affective empathy is the ability to feel what others are feeling emotionally, or at least to be emotionally moved by another's first-person emotional experience. These disorder, however, do not affect cognitive empathy, which is the mere cognizance of another's emotional state (i.e., the ability to "read" another person). Today, both disorders are grouped together in the DSM-V under Antisocial Personality Disorder, but they are indeed different. What makes them similar is their diminished affective empathy, which is what I believe you are referring to when you describe your moral ambivalence, or scruples. Neither psychopathy or sociopathy entail an absence or morals. And the affective empathy component of both disorders exists on a spectrum ranging from completely absent and up through varying degrees of diminished presence. In other words, it is usually not an all-or-nothing trait. Both psychopaths and sociopaths will possess varying degrees of affective empathy across a sample size. Regardless, neither sociopathy or psychopathy necessarily entail murder or murderous tendencies; however, most if not all serial killers are believed to have one or the other.
As I stated earlier, environment is inextricable from genetics (as epigenetic mechanisms are being constantly elicited by one's environment, and these mechanisms elicit phenotypic changes), but overt environmental factors can and do play a role in the development of paraphilias. For example, Dennis Rader (BTK) remembers first being turned on sexually by death when he was a young child watching a family member behead a chicken. Whether the seed was sown before then by abuse, sexual or otherwise, I do not know. But this paraphilia - his sexual turn-on regarding death, violence, and murder - certainly grew to the point where he decided to indulge it in real life. Often, these fantasies start small, but persist and grow into rape/murder/power fantasies, that they then decide to carry out for real. But, obviously, not everyone who harbors such fantasies carries them out. So what does? What is that final straw? I'm sure it differs by serial killer. Maybe desperation for some? Opportunity for others? But as with all human behaviors, they exist on a slippery slope once one takes that initial step and succeeds without falling (e.g., getting arrested). A more benign example is learning to ride a bike. Once initial failures (e.g., falls) are overcome, and riding becomes possible, it becomes easier and easier to do.
So, to answer you question, yes it is something you are born with partially, and something you developed environmentally, partially.
As for the soul. It doesn't exist. So, no, I don't believe it would be a psychological factor regarding the aforementioned disorders except as a schema through which an individual views the world; though possibly also, as in your case, an analog for a moral sense or moral "force".
(PS: If anybody has any corrections, please amend. I haven't studied this in a little while, I just finished lifting weights so there's very little blood in my brain, and it's past my bed time.)
