NPR has a fascinating, ongoing series of stories about neuroscience, crime and the law—starting out with a piece about a scientist who discovered a dark secret in his own brain.
James Fallon studies the biological basis of behavior, especially the differences between the minds of psychopaths and normal people. It's research that can produce an almost knee-jerk recoil, given that this kind of stuff was once used to justify forced sterilization and other eugenic practices. But Fallon's story actually ends up illustrating why you can't just write off people as "damaged goods", even if they do carry genes that might predispose them to violence.
When Fallon's own family history turned out to be chock full of murderers (including Lizzie freaking Borden), he started studying himself, and found that his brain scans match those of people born with a lowered ability to control their id-like appetites—from rage to food to sex. He also carries a gene that prevents his brain from properly using the calming chemical Serotonin—a gene that's associated with increased levels of aggression.
But Fallon isn't a killer. Or even particularly off-putting, according to the story. The point: What makes you you isn't shaped entirely by brain chemistry or genetics. We can say that there are inherited traits that seem to predispose someone to certain behaviors, but we can't say how that will play out in the real world. Biology is destiny. Except, you know, when it's not.