They thought "Well, I guess this makes me one of those people for whom "Not talking about politics with Friends" becomes a core tenent to my personal philosophy."

They thought that the union members were criminals.

This is about having political conversations with friends, not with people shooting at you with machine guns.

The original argument put forth by capitalists was that unionized workers were effectively engaging in economic sabotage by striking and blockading factories.

That said the Pinkertons were basically mercenaries akin to organized crime, so probably viewed things in terms of might makes right.

They thought, "if I do a good job, I'm going to get a bonus"

Unfortunately they're thinking the same thing today.