CRUELTY requires power to implement it. It does not take genius or intelligence. What leader of what country is most cruelly idiotic?

China. 

I think the way you posed this question misses a crucial distinction.

While it does not take intellect to commit cruelty, it DOES take intellect to impose cruelty...and to not suffer backlash or negative consequences from it.

We may not understand the goals of people in power who are despotic and abusive, but they do have them (often, that goal is simply to remain in power). We also may not understand the particular dynamics of the situation which prompts a despotic leader to impose the cruelty he does. But those dynamics do exist, and often the cruel despot understands the dynamics quite well.

Sometimes, the cruelty results from a mistake on the part of the despot. He (and it's most often a "he" historically) makes a calculated decision, believing he has all the relevant information, and it turns out some of that information was incorrect, or there were other factors he did not consider.
So that particular decision makes the despot appear to be stupid. But that is often a case of "Monday morning quarterbacking."

Hence, I don't think we can use the imposition of cruelty to determine how intelligent (or not) a national leader is. The cruelty he imposes may make rational sense given a leader's particular circumstances.