The world can't agree on one form of government, how could we agree to merge together under one rule?

Absolute power corrupts. Ruling the world is about as absolute as it could get for humans.

The sheer size of this brings up issues of span of control and the large disparity in needs/wants. The larger a collective gets the less they have in common. The larger a population the larger the disparities within the populations. Look at the size (population and actual geographical size) of the US and what issues arise. If the US has issues with how to distribute wealth due to the diverse population and size of land mass the entire world will have these problems in greater scope.

Resources are finite and as such there will always be inequality to some degree. The less people have in common with others and the less they are likely to help out. Empathy has limits, distance and differences play into this.

On the small scale people exist in a state of cooperation with the occasional member or group having greed or a larger share. On the large scale what one group wants puts it in direct competition with another, but allows for semi mutually beneficial cooperation opportunities between groups (trade for example). Lumping large groups of dissimilar needs and wants together would be hugely problematic.

An example of this idea on a grand scale having issues would be the failed Kyoto Protocol.

Who the fuck runs this?

What a bullshit idea, do you thing we're just all going to get along under the same flag?

that is ridicoulous because there would be more killings then ever before and more crime.

I like individuality so no. Also if there was only one government then if that government became oppressive then there would be no place people could go to escape it.