I do believe that it is within the right of the ICC to hold all leaders of the world accountable for their actions. While the world is comprised of many countries, I do believe that no one should be exempt from being prosecuted, for things such as war crimes, crimes against humanity, and genocide just to name a few. If one or more countries are exempt, it only takes one rogue leader to believe they are above the law and commit and continually commit horrific acts.
As technology advances, all manner of things improve: communication, travel, transport, and weaponry, to name a few. This particular combination makes the world a smaller, more dangerous place. Nations of all sizes (and even simply factions within nations) have capabilities now that they never had previously to exercise aggression, and they do. This gives rise to the need of a tribunal to hold leaders accountable for their actions. The ICC is that body. As anything else, though, it is rarely that simple. A court like this is necessarily fettered by politics, and it is questionable how much it can accomplish. Still, it must be attempted, and all leaders should be subject.