Strangely, a popular formulation of utilitarian ethics attracts utilitarian ethicists, some of whom were eugenicists and the like already by inclination, the rest merely having become so under the suasion of this fringe theory's false axioms.
When your scheme of rules for how human societies should run does nothing to exclude or even discourage the worst atrocities of human history - when that scheme is fairly evaluable on its own terms as declaring those atrocities insufficient! - you've already made a catastrophically terrible mistake. To advocate it thereafter through persuasion rhetorical and otherwise is contemptible, but unsurprising.
When your scheme of rules for how human societies should run does nothing to exclude or even discourage the worst atrocities of human history - when that scheme is fairly evaluable on its own terms as declaring those atrocities insufficient! - you've already made a catastrophically terrible mistake. To advocate it thereafter through persuasion rhetorical and otherwise is contemptible, but unsurprising.