This is a pretty popular theory argument among conspiracy theorists but doesn't hold much water in the sphere of the real world and policymaking. I assume you are talking about the US creating this global policing state, so that's what I will base my arguments off of but they can easily apply to other countries.

1. This theory completely assumes the existance of the concept of hegemony and that such a state (hegemon of the world) would actually be obtainable.

2. The theory assumes that the United States is the hegemon of the world.

3. It also doesn't account for the fact that in the theoretical world of hegemony that soft power (eg benevolent relationships with the other nations) is just as key as military hard power. To establish a position of hegemony would mean that the country would need to have good relations with the other nations as well as military power, neither of these can create hegemony alone. This completely takes out the fact of a "global policing state" as such a state that is being described is being achieved through the means of hard power alone.

4. Survival of the state is what justifies tyranny, not hegemony. Nations will do everything and anything they can to ensure the survival of their nation. In a means to "protect the Fatherland" dictators will justify each and every form of dehuminization, tyranny, and rights violations. So in fact it will not be the hegemon specifically being the tyrant, it will be the repressed states. To the point where survival of the nation-state is of upmost importance to the government they will justify every atrocity known to man under the guise of surviving. It is the oppressee, not the oppressor, that justifies tyranny.
lol.

You're making a lot of assumptions without knowing my full position on the subject.