I try to remain calm in those situations. The person I'm talking to is rarely the person causing my problem, and they are usually not paid well enough to take crap from me.
Another consideration is that you never know what else has been going on in this person's life. No need to pile on.
My final thought is something my granny often told us. "You catch more flies with honey than vinegar." I experienced this recently when I was waiting in line to resolve an issue at a store. The person before me was being a jerk. They were asked to step aside and wait for a manager. I calmly explained my problem, similar to what the jerk was complaining about. The associate quickly resolved my issue and gave me a $5 gift card for my trouble. The jerk witnessed this and his head was ready to explode. The associate just smiled.
Love that story!! And your Gran was correct.
We never know how powerful our actions are.
It reminds me about how everyone is afraid of going back in time as what they do could change the present. But none of us think that what we do in the present effects the future in the same way.