This post is mostly about collectivism and communism, and their flaws, but still, utopia fits in.
Every time someone tries to create a utopianistic society, it is only ever one execution away from the vision’s fruition. What do I mean? Well, think about it. Not eveyone agrees with the system, and in most systems trying to be a utopia, such as communism, the dissenter is the threat to the utopia, and his/her execution will be seen as pivotal to restoring the utopia, but then the utopia is never realized and still does not exist, no matter how many dissenters are killed by the state to support it.
Same with world peace, or peace in general. If one were to create a global government designed to bring world peace, said peace would still not exist, and not everyone would like said establishment. What if said dissenters were killed by the government, just like in any dictatorship, just to preserve the harmony and the peace, which, again, still does not exist. If you could really create true peace in any society, let alone the world, you’d have to get rid of anything that could potentially disrupt that peace. Anything. And you would still fail to create a utopia.