Arguably, if you have a choice of countries to 'sort out', it's logical to go for the countries who are oil rich first, as stabilising the worldwide supply of such an important yet dwindling resource is as good a reason as any to prioritise one country over another.
Given we (USA, UK, France, etc) can't 'sort out' all the world's murdering dictators at once, you have to take on the worst cases first.
A nasty dictator who has oil in his armoury trumps a nasty dictator who hasn't.
Simple as that.