Here's one way to look at it: very, very smart criminals should do much less damage to their victims than they seem to, just for reasons of self-interest. This is primarily because the first world has made it more individually profitable for very, very smart criminals to go legit, either in business or politics. This leaves actual visible criminal activity to those not smart enough, or not disciplined enough, to take that route.
Don't shake your head at how awful it is when a high-up executive at a major corporation turns out to be a complete sociopath. Marvel that society has gotten so much good out of that clever, ambitious sociopath. And be glad that crime is emphatically not as good a career path for him.
The article also doesn't explain why government officials, even corrupt ones, fail to make cheap investments that pay off over a few years. For instance, a kleptocrat should want a reliable supply of clean water to ensure that the farmers are healthy enough to plant and harvest crops, so he can steal the profits. If an epidemic means the crops rot in the fields, that's money out of his pocket. Money is readily converted to power, via bribes, weapons, or propaganda. Losing money means losing power. Why wouldn't a kleptocrat spend money on sanitation, especially if that means getting good marks from foreign governments and NGOs and qualifying for more aid?
It's probably simply too unstable to do so right now, and a lot of that comes from the top.
And that would be a lot of the difference between Cameroon and a poor-ish but more culturally Western country.
This is not to imply that the nobility always _did_ oppose the enemies who they should have been opposing; but it's not at all the case that had there been no feudal militaries, medieval Europe would have been a peaceful place.
(I acknowledge that this is a very late post, and I apologize for that; but it's a common mistake to imagine that medieval European history occurred in a vacuum. The primary culprit here is the, ah, _insular_ way that English-speaking schools teach medieval history...)
And I was specifically talking about the Western European nobility between, say, 1000 - 1300 AD, a period when there were no outsiders invading Western Europe and it was the nobility itself that posed the greatest threat to the lives of "commoners".
I do, by the way, take issue with your description of the Muslims as a threat - that was true only between around 650 - 750 or so, until Charles Martel stopped their expansion at the battle of Poitiers, and it was true again for Romanians and Hungarians in the sixteenth century when the Turkish empire started to expand westward, but between those times (a period commonly known as "The Middle Ages" ;-) ) it was Western Europeans that were threatening muslims by travelling to Palestine in a misguided attempt to "liberate" the "Holy Land" from the "Infidel".