We always prefer to have simple answers to complex situations, and this is no exception.
People aren't born violent or evil. If they were, then it would be a fault of genetics.
The culture we all grow up in plays a major role. Many things we are exposed to help shape who we become, but every recipe is different.
Our brain gets wired up in early childhood development, and it's vastly complicated, even for children who grow up in the same household. Perhaps one was bullied in school, or fell in with the wrong crowd, or internalized matters in a different perspective. Even siblings aren't treated equally.
I think much of violence and aggressive behavior comes from anger, lack of learned emotional regulation, poor self-image, drug experimentation, and damaged morals, values, and ethics. For some, it's a time bomb waiting to go off. It seems to me that the death penalty is assigning blame entirely to the individual, and that who they have become is 100% their choice. I used to think that because it was convenient but have since learned it's complicated.