"Unions exist" not 'were created' or 'have always been'. Yes, some Unions have fairly unpleasant histories as do the employers that they were organizing against. Unions have typically been much faster to offer protection to marginalized groups than employers or even governments have. My mother's Union protected her back in the 80s when it was still perfectly legal and commonplace for people to be fired for being gay. My immigrant father's Union protected him in the 70s from bigoted management. My union has been offering protection for transsexuals for decades.

Union violence was often a direct response to the violence of employers. Employers' casual disregard for the safety and lives of their workers undoubtedly outstrips any violence perpetrated by Unions. Let's also not overlook the violence of their paid union busters either. Thankfully Unions have moved past that despite so many employers ignoring or circumventing safety standards and sending their employees out with missing limbs or in body bags.

And if you're going for historical accuracy, it's very likely that the formation of Unions would have been impossible without violence.