Do you think there would be less conflict in the Middle East if everyone had the same (lack of) nationality? Unless you are willing to engage in mass genocide, eugenics and eternal subjugation to achieve your world of peace, there is little you can do about people that look like each other and live close to each other from identifying as a single unit. The idea of social constructivism has deluded people into believing that any human problem can be fixed by removing words from the dictionary. It's absurd.

Why would you want such thing?

Because it serves no purpose but strife, hatred and intolerance of other human beings?

Those things existed before and without nations. Civil wars happen despite the participants being of the same nationality. A country is (usually) simply a label for a group of people with a shared culture, language, mindset, etc. Removing those labels is a naive attempt at achieving some global utopia filled with uniform drones.

And how would doing such thing stop those who hate and are intolerant?

It would reduce the justifications such people have for their heinous activities.