Hi, Gabrielle, welcome to the forum!
That's an interesting question, I agree. It's hard to know what causes a person to become a serial killer--from what I've read, I think Bob is right, it's kind of a mixture of "nature" (biology) and "nurture" (the environment in which you're brought up).
I can see your point about the inbreeding--it did for sure happen a lot in royal circles, cousins marrying cousins, since the number of appropriate princes and princesses was obviously pretty small to begin with. A perfect example is the hemophilia that plagued so many royal families around the time of the First World War--it apparently began with Queen Victoria, and since so many of her children intermarried into other royal families, the disease (which is hereditary) spread far and wide in royal circles.
My dad, who has a degree in genetics, used to tell me how it was a very bad idea to breed two animals that are too closely related (whether humans or other critters!) If there's a hereditary disease lurking anywhere in the family tree, and two closely related people (or animals) mate, it greatly enhances the odds of the disease making an appearance.
Much healthier to marry "outside" the family--you enlarge the gene pool that way, which is a good way of avoiding these hereditary problems. Offspring tend to be stronger, and healthier--something geneticists call "hybrid vigor".
Am I sounding too much like a teacher lol?
Sorry--end of lecture!
But you do raise a very interesting point!!
Oh, and I agree with Bob--"Whoever Fights Monsters" (written by former FBI profiler Robert Ressler) is an excellent book. I've also recently read a similar book by a former profiler which I highly recommend: "Profiles in Murder: An FBI Legend Dissects Killers and Their Crimes", by profiler Russell Vorpagel and Joseph Harrington. It's a truly fascinating and informative book!