Slackbridge is the union representative sent out to organize the workers at Bounderby's factory. His main way of whipping up support is to blow everything he says completely out of proportion and to use many words quickly to lull his audience. In one speech, he demonizes Stephen, calling him out on the fact that he won't join up with the other workers. This is a pretty good trick, since by all banning against Stephen, the workers end up on the same team together, and are thus much more likely to unionize.
Like the novel's other liars (Tom and Bounderby spring to mind) Slackbridge doesn't have too many redeeming qualities. And yet, the reader is definitely meant to feel sympathy for the workers. The conditions of the factory are clearly horrible. So, why create this kind of contradiction here? How would the strike plot be different if Slackbridge were a positive figure? If the unionization was being organized by one of the workers rather than an outsider to the town?