I would like to see more then just one ethic set up to be able to choose the fanatical purifier civic.
Authoritarian/ fanatical xenopbobes sounds logical.
Maybe they didnt wanted to brandmark authoritarians automatically "evil" as it was the case with collectivists (best ethic for slavery and purging).
The authoritarians are even more slavery focused than the collectivists it would seem and anyway it is still primarily the xenophobe part of the ethos that would be responsible for the fanatical purifier personality.