The danger of Utopians believing that they have the plan for creating a perfect world is that they are likely to attempt to force people to cooperate in creating their "perfect world". If Utopians are successful this results in enslavement of "the people" whose lives they are supposed to be improving. Utopians are likely to view anyone who interferes with creating their Utopia as an evil enemy. As a result Utopians are likely to kill those who oppose them. Large scale massacres have been committed by Communists in the quest for a perfect society. The Nazi's in their quest for the perfect Aryan race enslaved and massacred those they did not consider perfect. A country in which people are enslaved and killed is hardly a Utopia.
If Utopians succeed in turning their country into a "paradise" they are likely to attempt to export Utopia to other countries. The Chinese Communists for example wish to incorporate Taiwan into their Communist empire and have already incorporated Tibet.
Communism is discussed further on another page in this web site.