I was basically yelled at for doing my job by a coworker who can't even do his job right. Each time I try to explain myself to him, I have to cut myself off to avoid starting an argument. HOW DO YOU EVEN?!
You said co-worker not supervisor. You have no need to justify yourself to anyone else that isn't your supervisor. That comes from a retired Supervisor of many, many years.
I used to have a very difficult co- worker. I had to see a psychiatrist for job stress. The shrink asked me, "Can you tell me in just a few words what is stressful about your job? "
I answered, "I can tell you in TWO words: Frank H_____." The shrink signed and said, "You know, I've been asking patients that question for many years now and it's amazing how often they answer with a name."
My problem solved itself when the guy got fired for writing a very nasty sexually harassing letter to the director of the institution in which I worked.
In retrospect, I probably should've taped him and sent it to someone in authority but at the time I feared that I might be fired if I used surveillance equipment which was against the rules where I worked.