^If their motives and choices were entirely comprehensible in human terms, they wouldn't be aliens. That, too, was part of the point. Sometimes coexisting with another culture requires broadening your standards of what constitutes a morally forgivable act. How else did the Federation learn, later on, to become allies with the Klingons?
Here you just try to excuse lazy writing with an ~"it doesn't make sense - but that's OK".
The gorn were unequivocally in the wrong. Any other interpretation de facto gives them carte blanche to commit mass-murder because 'it's their way' and therefore morally OK.
If that constitutes a 'morally forgivable act', then ANY act is morally forgivable. Including, for example, a dictator exterminating earth, or some thugs killing your family because 'it's fun', etc - after all, it's their way, yes?
If you push your moral relativism to this point where everything is morally OK because 'it's their way', then your morals are worthless; indeed, you have no morals beyond acceptance of being trampled by anyone, Christopher.
And the intent of the scenarists was NOT to depict the gorn unequivocally in the wrong - not even by human morals - that's why they put the part with the main characters having doubts about their actions, etc.