For Claire, an intelligent liberated woman of the 20th century, such barbarity is unacceptable, indeed contemptible. But, stripped of modern ethical standards, clan society in the 18th century worked pretty well, and Claire was operating in a cultural vacuum (her culture had not yet been born!).
The episode put me in mind of Alistair Moffat's comments in his excellent book on Hadrian's Wall, on the relative barbarism of Romans and the invaded, artistic but illiterate Celts, "In AD 105 the Emperor Trajan sent 50,000 captives back to Rome to be butchered by gladiators for the amusement of spectators... very civilised".
Where a culture has superior military power, it somehow believes that its values are superior to those who are less developed, less able to defend themselves.
In 1919 the Aliab Dinka of Southern Sudan, naked, spear-carrying cattle herders were not paying their taxes and, when confronted, had the temerity to outwit the government forces and kill the provincial governor. The Lewis gun equipped punitive expedition burned villages and drove off 7000 cattle, sold to fund the occupying force. Who were the Barbarians?
The extirpation of a cult was more or less exactly what the Duke of Cumberland had in mind with his brutal and indiscriminate suppression of the Highlands in 1746. The violence and sense of superiority of the British Army of the day is only a little exaggerated in the TV series.
That was 270 years ago. But the conviction that more developed cultures are superior (and should be imposed) still endures.