They took a calculated and premeditated swipe at the seat of power. Even a liberal democracy built on loftier principles than Machiavelli's cannot afford to let this slide.
The mob's purpose in the the plan devised by Eastman and enacted by Trump [1] was to serve as a distraction / excuse for Mike Pence to launch a contingent election with fake electors who would certify Trump as the victor, thereby terminating our democracy and ending the American experiment. Pence refused to do this and that's why the peaceful mob started chanting "Hang Mike Pence, Hang Mike Pence."
I have to admit, before that point in time I saw Pence as the diametric opposite of every political belief I ever held -- because I did not realize that a basic belief in American democracy was up for question. I thought it went without saying that we all shared that value. We did not. But Pence did, and he set an example of "Love Thy Neighbor" that I will not soon forget.