To be frank, why would Kirk be to blame for this? The Klingons armed the other tribe first, who then shot at Mr. Spock and wounded him. Kirk was trying to broker for peace, but circumstances (and the oversexed, backstabbing wife of his friend) forced his hand. Plus, Kirk didn't give the Hill People overpowering phasers, but standard flintlocks equal to the other tribe's. It seemed to be a complete and utter mess for all concerned, and not a straight Prime Directive violation like the kind that Kirk usually does.