Though very intelligent and capable, he's obviously not truthworty and frankly a a sociopath. He's probably one of the better choices among the villains of the show, because he's certainly SMART emough to rule, and he's at least better at HIDING the fact that he's a monster than people like Dany or Cersi. But ultimately, he's another monster on the throne who would need to be killed sooner or later for the greater good.