Imagine there are two kingdoms on some continent. One is managed by some kind of democratic council, and resources are shared equitably; the other is ruled by a tyrant. Which one is more likely to make war on its neighbors? The tyrant, obviously- because the tyrant has something to gain by conquest (more subjects, wealth, and power) and the democratic society does not (assuming that the society stays democratic after the conquest, they now have to contend with the voices of a lot of angry new citizens). The same applies to any organization: an organization is aggressive and violent in direct proportion to how hierarchical it is.
There is a tragic principle of natural selection that plays out, however, if you imagine that continent and imagine a random arrangement of democracies and kingdoms, over time the kingdoms will make war and expand and the democracies will not. After a few hundred years, most of the citizens of the continent will be living under tyrants. And tyranny has to justify itself to its subjects, through religion or ideology; the subjects will be made to feel that obedience is virtue and that the king is a father figure and so forth. But popular freedom movements will constantly be popping up and have to be suppressed.
I give you: the modern world.