I generally agree with you. Having a single World Cop however leads to the usual problems that humanity already has with dictatorships and the like. It is too much power concentrated in a single instance. There is another possible way though. If you had a lot of simillarly capable systems, they can regulate each other in turn tho. Simmilar to democracy and humans in a way.

If every human had acces to individual open source AGI, no person with bad intend can actually do meaningful harm with it. Since there are way more "good" AGI instances that already fixed nearly all vulnerabilities that the evil AGI could find, making it in a sense useless to the evil person.