The Gorn weren't villains in TOS. The whole point of Arena is that the Gorn weren't evil, just caught up in a massive misunderstanding.
Mmmmmm.... I don't know if I agree with that. Cestus III was set up by the Federation, ostensibly as a peaceful research outpost. One would think that if there was a Gorn colony there, or ANY indication of a non-Fed presence in or near the system, they would have kicked in first contact/diplomacy protocols immediately. Prior to the appearance of the Gorn in SNW, there was no indication that the Federation ever even heard of them until "Arena". We can probably easily assume that there were regulations in place against plopping a colony down on an already-claimed world by another power, unless prior permission was specifically given. There was no evidence of any of this happening, at any time.
So, either the advance Federation charting teams EPICALLY dropped the ball researching that area of space, or the Gorn claimed a completely uninhabited world, left behind no indications of such a claim, and got pissy when they discovered that someone else unwittingly moved in, leading to a MASSIVE war-level overreaction by the Gorn.
Not to mention deep-faking outpost personnel messages to LURE the Enterprise in to blowing them away, too. That was not self defense - that was straight-up war-crimes murder, leading them quite handily into "villain" territory.
From
Trek Transcripts:
I understand and appreciate the emotion of territorial protectionism, but being that way and NOT putting up a sign saying "GO THE HELL AWAY!", however (like what the Tholians and Melkotions did, for example), the onus is pretty much
all on the Gorn for this one.