The King of Brobdingnag is horrified to hear the stories that Gulliver tells of England. The people of Brobdingnag are peaceful, and they do not engage in war if they can avoid it. He concludes that England must be full of barbarians, and that the leaders are not qualified to rule.
This reaction is probably a veiled representation of Swift's own opinions of England. Swift is likely to believe that England is indeed too violent and that its leaders are not fit to rule.