(3) The way unions draw lines -- you're in the union, or not -- inevitably breeds an "us vs them" mentality. So a union "brother" who's a thug and beats people up is a good guy, but a "scab" who otherwise might be a perfect saint is a bad guy. And the company is always the enemy. Thus unions rationalize violence. Also, thus unions wreck one industry after another, protecting themselves against everyone else, even driving their own employer out of business.
As long as unions gain their power through "union," it is inevitable that in they end they will become destructive. When (for example) the teachers' union protects itself at the expense of the kids' education, it's just doing what unions inevitably do.