Aight, I'm far from the first person to defend Enterprise, but I just watched the silo scene and there was a little more going on than is being put forth here. The Klingon was being pursued by the Suliban (whom the farmer didn't see), was armed, and had just blown up the silo with his weapon. The farmer tried to get him to drop his weapon and only fired when the Klingon started approaching him without doing so. So not exactly some potentially friendly alien who just crashed on the farm and got shot on sight.
The Question is: Why would a farmer guard a grain-silo armed to the teeth on a planet where supposedly hunger, poverty and war were things of the past? A grain-silo, of all things!!