"Prig" is in the eye of the beholder. What about when the "prigs" were right? I'm sure the Quakers were seen as "prigs" by the southern slaveholders/traders. The Quakers were early to the abolition party and their opposition to slavery was based on religious zeal which made them seem like "prigs" to the people in the South who's whole society and economy was built on slavery. But we now consider the Quakers were righ…
I think the basis of his arguement is a prig is incentivized by calling out the moral failures of others to make themselves feel more virtuous. Where perhaps the quakers or MLK were doing it out of moral outrage.
I would suggest instead that a prig deems a person to be bad/evil based on them having a different view/behavior that society is generally divided on.