Thinking of a united world right now seems more like a slogan from a politician. I also think that, as humanity is right now, a united world is not achievable. The possible solution for a better and more fair world is get organized in communities where people can choose the one they comfortable with.
This is the viewpoint of @adamkokesh. No central government ruling over people where freedom is the underlying pillar. I think this concept is more valuable.
Also, there's a dangerous aspect of a united world. Some people in power could use it for their own advantage. They right now do what they want. So let's just imagine if they participate in the construction of a united world! That would be terrible because they would make sure once and for all that there wouldn't be dissidents for any fair cause.
You are right, as a united world, we would end up a target for unscrupulous leadership to manipulate. It would end up where anyone who dared to say they disagreed would be eliminated.
Divided we still have a fighting chance of creating a world, or communities that are pockets of freedom if you like.