Earlier quoted context omitted.
Are you comfortable being judged on the worst thing you've ever done in your life? Sure, a violent crime is worse than some bone-headed mistake made at a college party, or whatever your worst moment is. But how much have you grown up since then? Would you do it again? Or are you older and wiser? Well, they are probably older and wiser, too.
I'd be willing to be that the worst thing most people have done isn't murder or manslaughter. You don't have to be "older and wiser" to know that you aren't supposed to murder people.
I mean, I'm not sure how relevant "knowing" anything is, but age is an incredibly strong predictor of criminality. Gender is another one. The majority of crime, and the overwhelming majority violent crime, is committed by young men. One factor that likely contributes to this reliable statistical phenomenon is that the prefrontal cortex, the part of the brain responsible for decision-making, self-control, emotional self-regulation, and other aspects of executive function, doesn't fully develop until around 25 years of age. For one example of how executive function relates to criminal behavior, look into the connection between ADHD (a disorder linked to deficiencies in the prefrontal cortex) and crime.
I agree that most people haven't killed anyone, but I also don't think that there's any reason for any of us to assume that 'we' would have behaved any differently when placed in the circumstances of those who have.