My experience has been that most union reps, even the militant hard-liners, come into office hoping to be able to forge some sort of working relationship with management. What inevitably happens, though, is that management throws it back in their faces, and then the animosity starts.
You also have to ask yourself, if someone that has animosity towards management is elected, why was that person elected by his peers? It's almost certainly because the employees that elected him feel that management is screwing them over, and they want a hard-liner to teach the company a lesson. Militants don't get elected for no reason. It's almost always because the company did something to deserve it.
I always point to SWAPA as an example of how this is supposed to work. For decades, SWAPA has functioned very well with management, because management hasn't done anything to cause the pilot group to get pissed off and elect militant reps. You can bet, though, that if management changes their attitude to one of hostility, the pilots will respond by electing more militant reps that will take an equally hostile posture towards the company.
It all starts with the company. Hard-liners don't get elected for no reason. They get elected because management did something to deserve it.