I LOVE this question so much! First of all, in our industry the SELLING agent, is the one who represents the buyer. The LISTING agent is the one who represents the seller. I know exactly what you mean and I found this confusing at first, but that's just how they have delineated between the 2 types of agents.
I had an agent who, when I was actually representing the sellers in a transaction, would belittle me, swear at me, hang up on me. I had to demand that all of our communication be via e mail or text so I could have it in writing. I worked VERY hard to keep everything professional. I did alert my broker to this behavior and, if needed, he was ready to jump in.
If this is a house you REALLY like, I would let your agent handle it. After all, that's part of an agent's job, to deal with such difficulties!! But, if you don't like the house or the deal falls through, it's well within your rights to contact the broker of this other agent to express your concern. It not only show support for the professionalism of that agent, but that kind of report will remain with this agent's information. It might not be a bad idea to submit this information to the Department of Commerce, too.
Unfortunately, some overly aggressive agents think this is the way to prove to their clients that they're working hard on their behalf, when it couldn't be further from the truth! To create an adversarial relationship in the course of negotiations, only serves to alienate the other party and potentially compromise the position of your own client.