“What I loved about the Gorn was it was an opportunity to retcon something into a real monster. What we do in Star Trek—and you’ll see we’ll even do it with the Gorn—is we start by seeing the other and often we end by engaging our empathy and understanding common ground. And that’s great, and it doesn’t mean that there isn’t real evil in the world. And so what we wanted to do with the Gorn was to give you a monster, and a monster that at least at first, seemed irredeemable.”
I find this statement a little aggravating, because in my opinion they really haven't retconned the Gorn. In "Arena", they slaughtered the entire Cestus colony, and the Gorn captain is utterly merciless.
"Arena" is about mercy, but the Gorn didn't really earn it in the episode, which was really kind of the point.