Are all dictators evil

No although it is hard to believe the best governments are run by dictatorships. The dictator must keep a strong democracy among the people without taking advantage of his or her power.

Why are all the worlds superpowers a democracy or republic?

Although dictatorships work the most effectively one bad dictator will take power and destroy all the previous dictators work. This is also why dictators get such a bad reputation. Although a republic or democracy can not function as well its progress cannot be destroyed.