Here of course one has to deal with the inevitable objection: that utopianism has lead to unmitigated horror, as Stalinists, Maoists, and other idealists tried to carve society into impossible shapes, killing millions in the process.
This argument belies a fundamental misconception: that imagining better worlds was itself the problem. Stalinists and their ilk did not kill because they dreamed great dreams – actually, Stalinists were famous for being rather short on imagination – but because they mistook their dreams for scientific certainties. Thie led them to feel they had a right to impose their visions through a machinery of violence. Anarchists are proposing nothing of the sort, on either count. They presume no inevitable course of history and one can never further the course of freedom by creating new forms of coercion. In fact all forms of systemic violence are (among other things) assaults on the role of the imagination as a political principle, and the only way to begin to think about eliminating systematic violence is by recognizing this.
And of course one could write very long books about the atrocities throughout history carried out by cynics and other pessimists…