I just posted to a slashdot discussion about how neuroscience appears to be eroding the notion of free will. Some really interesting philosophical threads there that touch on tangents in physics, biology, and religion.
The genetic angle on this topic comes up in the field of sociobiology, which ties into what I mentioned at the end of my slashdot post. Genes create predispositions for certain behaviors, many of which impart a selective advantage. So many of our behaviors, at a coarse level, are in place since they helped our ancestors survive. At a fine level, evolution doesn't shape our specific, day-to-day behaviors, but it certainly channels us into certain predictable directions.
The physical structure of our brains is necessary by not sufficient for all the specific behaviors we have or are capable of having. Borrowing a concept from genetics, one could say that significant alterations in brain physiology by things like tumors, are likely to have pleiotropic effects on many things that depend on brain function, such as all of our characteristically human behaviors.
So given this, we should not be so surprised when a tumor is proven to have been at the root of someone's decision making process. I guess it shakes our world view regarding what it means to be human.