: "They came in space normal speed, using our regular approach route, but they knocked out our phaser batteries with their first salvo. From then on we were helpless. [..] They poured it on, like, like phasers, only worse, whatever they were using. I tried to signal them. We called up. Tried to surrender. We had women and children. I told them that! I begged them! They wouldn't listen. They didn't let up for a moment."
So pretty obvious that they made themselves known before the attack. And the starbase was defenseless after only the first salvo took out the base phaser batteries. There was no need to continue destroying, and yet they did. I don't know how the communication issues were, if they understood or not, but the universal premise of respect for life is not hard to appreciate and follow. They could have taken prisoners. The indigenous population wasn't military but civilians, so there was no need to wipe them out. To me that depicts a ruthless bloodthirsty species.
GORN [OC]: You were intruding! You established an outpost in our space.
KIRK [on viewscreen]: You butchered helpless human beings
GORN [OC]: We destroyed invaders, as I shall destroy you!
MCCOY: Can that be true? Was Cestus Three an intrusion on their space?
SPOCK: It may well be possible, Doctor. We know very little about that section of the galaxy.
MCCOY: Then we could be in the wrong.
SPOCK: Perhaps. That is something best decided by diplomats.
MCCOY: The Gorn simply might have been trying to protect themselves.
KIRK: No. No, I won't kill you. Maybe you thought you were protecting yourself when you attacked the outpost.
(He throws the dagger away, stands up and shouts to the sky)
Meh... that was a pretty weak argument. "Protect themselves?" No, it was the territory they were seeking to protect. No Gorn lives were threatened or lost. They could have taken out the phaser batteries, landed, then forced everyone to leave the planet in whatever vehicles they had. But that isn't their way. A question of civility.