>Why did they kill the other goblin when they saw the fence?

I´m guessing he was trying to warn them that something fucky was going on. If you look at him closely, you´ll notice he was also the only one smart enough to try to camouflage himself somehow by wearing leaves and twigs on top of his head.He might have been one of those cases of "guy in tribe of idiots that gets shat on for being the only one with a brain".