In their composite measure, they take a look at years in power, percentage of GDP dedicated to military spending, prison population per 100000 people, press freedom and a subjective "expert score" to rank these rather nefarious characters. While I see the logic in using national-level indicators for the most part, they have their limits. This consideration is especially true for those countries which have relatively newer leaders.
A case in point is Kim Jong-un. The WPI folks do a shortcut by adding +3 years for tenure for those who inherited their leadership from a family member. While little may change in North Korea, there is certainly the possibility of reform on the margins--or even more extensive changes. For instance, see Parade's 2011 rating in which Myanmar's Thein Sein ranked fifth largely on the strength (weakness?) of his predecessors' actions. It took some time to get going perhaps, but he was intent on reform. We'll see: let's not give up on Kim Jong-un just yet. Sins of the father and all that...
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