The problems were created by Republican policies, in support of corporate interests, and conveniently (and effectively) attributed to those opposing said policies.
They said the same thing about the civil rights movement, to promote increased suppression and segregation of black minorities.
"Democratic" and rights-based demos typically begin peacefully. If violence occurs it's usually induced by opposition attacks or overzealous policing.
The left tends to target symbols of oppression or grievances, not people. It's the right than more often resorts to violence, indiscriminate attacks, and violence against people.