My sister is a psychiatrist in a facility for the criminally insane. A high security prison for violent criminals. She is also a Catholic.
Some people are sociopaths, seemingly from birth. A brain deformity or mental condition perhaps due to birth trauma, or an abnormality the person is born with, just as others are born with physical malady or mental retardation.
Many of these people do have horrific childhoods, often including the sort of physical abuse that would physically injure a developing brain. Shaking, beating, punches to the head, small children being physically thrown against walls etc. But some come from seemingly normal families, and showed bizarre and aberant behavior even as young children.
In other words, they were never quite right.
Anyone who has raised a child who has significant brain issues will understand this. It can be horrifying, heart breaking and baffling, as a seemingly physically normal child, in a loving family clearly has mental issues that are there from birth. Some of these children seem incapable of forming normal human attachments, or understanding or caring about the feelings of anyone around them.
So, can’t definitively say that serial killers are born that way, but some people are born with brain function and personality disorders that predispose them to anti social or violent behavior. It is not truly possible to know how much, if any choice they have over how this develops and manifests during their lifetime.
But as people can see that babies are born with all manner of mental and physical deformities, we can see how at least the serious seeds of this behavior can be an issue they are born with.
I have three children. One is autistic. Intelligent and raised in a loving family, but he has this issue he was born with that significantly impacts his interaction with others and his ability to form relationships. He in not profoundly autistic, but in the course of therapy for him, we met with families that had profoundly autistic children.
My brother, a Catholic spiritual advisor, used to do volunteer work with families of children with profound brain/behavior issues. Children who literally could not be left alone or unsupervised EVER, because of self harming, or violent behavior. This in 3 yr olds. Certainly not any choice they or their families made.