Let's assume that utopia is possible, or if it’s not, let's instead imagine the state closest to utopia. Nobody would assert that today's world is that utopia. In fact, my suspicions are that it is impossible to ever arrive at that utopia via incremental steps.
One essential property of a utopia must be stability; it must be robust to degeneration into a state of non-utopia. I believe that achieving this single property requires the elimination of several unbalancing factors in today's world, as there are simply too many causes for conflict that would otherwise result in a collapse of the utopian society.
I then considered the design of an ideal society, without constraint. One important feature must be sustainability. Several ills can be attributed to the over-extended reach of humanity, which results in sub-optimal assignment of resources. If the population is to be distributed and controlled according to reasoned principles, then human numbers would be small, but localized to the most habitable regions where resources are ample and the incidence of natural disasters minimal.
My suspicions are also that social and familial structures would be different under a utopia. Social and familial structures are institutions founded on principles and assumptions that may not be the most optimal, but are merely too costly to change en masse.
A final, though dangerous, thought is that the people of an ideal society must themselves be superior to modern humans, not only in manner and philosophy, but perhaps, also in nature. There may be several genetic traits most conducive to an ideal society. Two properties that should, if possible, be selected for are empathy and restraint. Other physical advantages, such as resistance to disease or improved physical and mental abilities, are also boons that are almost certainly beneficial.
These lines of reasoning lead me to believe that it is difficult to form an ideal society, due to inertia and resistance. It is not possible to change social structures overnight, nor can the world population be reduced or redistributed. Incremental changes are simply not feasible.
Granted that the utopia is stable, any utilitarian must surely conclude that it is acceptable to usher in such a golden age via any means. Imagine if a dark age were declared specifically for the execution of 'any means', and any atrocity pardoned if it follows the grand plan. Utopia should then be treated as if it were founded on a blank slate.
The sole weakness of this idea is that it royally screws the present for the eternal future, which is why Utopia will never be realized.