The fact that it was legal, in a democratic and law-based society, was the thing that was peculiarly horrible about it. In the same way there is something more horrible about a state-authorised pogrom than a race riot.

It's one of the reasons why Nazi Germany is such a chilling precedent - it was in many ways formally a democaracy. The government was only there because of the results of an election, and parliament and the courts were all still in place.