He says what everyone pretty much knows about why serious socialism leads to mass murder, but he uses the Trolley Problem in Ethics 101 as an image:
We can see that calculus behind the utopian logic in the now famous ‘trolley problem’ in which most people say they would be willing to kill one person in order to save five. Here’s the set-up: you are standing next to a fork in a railroad line with a switch to divert a trolley car that is about to kill five workers on the track. If you pull the switch, it will divert the trolley down a side track where it will kill one worker. If you do nothing, the trolley kills the five. What would you do? Most people say that they would pull the switch. If even people in Western enlightened countries today agree that it is morally permissible to kill one person to save five, imagine how easy it is to convince people living in autocratic states with utopian aspirations to kill 1,000 to save 5,000, or to exterminate 1,000,000 so that 5,000,000 might prosper. What’s a few zeros when we’re talking about infinite happiness and eternal bliss?
Michael Shermer, “Utopia is a dangerous ideal: we should aim for ‘protopia’” at Aeon
The Trolleys were—and are—unleashed on anyone getting in the way of utopia. But what innovation does Shermer think will be better? Ah…
Protopia is a state that is better today than yesterday, although it might be only a little better. Protopia is much much harder to visualise. Because a protopia contains as many new problems as new benefits, this complex interaction of working and broken is very hard to predict.
Michael Shermer, “Utopia is a dangerous ideal: we should aim for ‘protopia’” at Aeon

Stop right there, Michael. We might disagree as to what’s working and what’s broken anyhow.
For example, it’s unsettling that he includes “animal rights” as a protopian achievement.
Take the guy who is going to court to protect his right to keep his Personal Support Pig in his dorm room. Joined by the girl who just somehow needs her Personal Support Scorpion in her group home.
They see local animal control bylaws as “broken”; the rest of us see them as “working,” upholding our right not to live side-by-side with pigs and scorpions.
But what most of us haven’t envisioned yet is that pigs and scorpions would acquire the legal right to live side-by-side with us… That, presumably, would a protopian achievement… Okay, it’s not mass slaughter but … oh well, another day, another naturalist/materialist pseudo-topia, it seems.
Shermer’s most recent book is Heavens on Earth, on which the essay is based.