No, they were not.
A key point in that episode is the possibility the Federation has (unintentionally) invaded Gorn space by settling on Cestus III, and the Gorn reaction, instead of being just that of "alien monsters," was of a people defending themselves.
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 III 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.
And how Kirk proves humanity is more civilized than the Metrons believe...
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)
KIRK: No, I won't kill him! Do you hear? You'll have to get your entertainment someplace else! (The Gorn disappears, and a young blond boy in a white shift appears instead.)
KIRK: You're a Metron?
METRON: Does my appearance surprise you, Captain?