How are the regime able to do this? Do a majority if Iranians support them? Too afraid? The only job is the government job? Why choose to partake in the massacre even if you are on Team Ayatollah? Do those guards not consider the people the kill as Iranians?
They have a large, very capillary police-like force that answers to the national government, trained to have no problem with killing people.
The Iranians have been protesting that force in one way or another for more than a decade.