I'd say "You know what? We're not perfect, but if you could see how far we've come? 100,000 years ago, we were killing each other. Constantly. We lived in little tribes, and if we came across another tribe that was smaller, we killed them. If we came across one that was bigger, they killed us. That was all we did. We were killers through and through. But we grew, and we discovered language, and laws, and trade, and we learned that you don't always have to kill the other guy. And we settled down, and formed towns and cities and nations. We created art and music. And yes, we still killed each other, but here's the thing. We started feeling bad about it. And we started doing it less. And with each passing century, we killed each other less and we felt worse about the killing that we did do. So by the time we got to the twentieth century, which even though it had our two biggest and bloodiest wars ever, it was still the least bloody century in our history (seriously, I know it's counter-intuitive but it's actually true). We're getting better. We are getting so much better. With every decade we become wiser, more humane, more decent, less violent. And it doesn't feel like that to us because the wiser and more humane we become the more glaring our faults seem to us. We always think we're getting worse, when in fact we're getting better. And we will keep getting better. And that is why we deserve to live. Also, we have nukes and we don't like being threatened."