The protesters actually started the violence. But the square is the only thing Americans seem to care about. The people there were not “rounded up”, those who died could simply have walked home.
Before the outbreak a the square, the government called in troops from “rare ethnics” - with the assumption, they would have no quarrel firing on Han Chinese people who were the majourity of the protesters.
When those troops arrived, in the outskirts of the city: they got immediately attacked. Dozens of tanks destroyed and hundreds of soldiers killed. Many bound to fences etc. and burned alive with gasoline.
Of course the protesters on the square did not know that.
From the point of view of the Government: that was not a protest but an uprising or insurgency.
Look at this for instance: https://simple.wikipedia.org/wiki/January_6_United_States_Capitol_attack#%3A~%3Atext=back+our+country."-%2CStorming+of+the+Capitol%2CCapitol+Police+during+the+storming.
A little bit more bad luck, and it could have ended similar.
That the protesters kind of got rounded up, encircled and then systematically got killed at the Tiananmen Square: is a myth. It did not happen.
They need them to breed the other terrorists they like to bomb.