The situation you describe is different and actually proves my point because Mr. Dahmer, presumably lives in a nation, and in that nation in which he lives there must be some form of state or organization that is imposing that law. Even if he disagrees they can impose the law on him through some form of force. He can run, he can fight, but the organization is bigger and thus more powerful than him and as such can make him follow the law or force him to face the consequences. See also why billionaires may be above the law in some places: they are individually richer and more powerful than the organization that would hold them accountable.
On the international level, if a group of nations declare that invading other countries is a crime but 1 of them disagrees and starts an invasion, the crime can only be punished by force, ie war. You could impose penalties on them too, which is a type of force or power, but look at Russia to see how effective they are.
