Re-asking a professor if I can write my thesis after I chose another one I am currently a student in computer science. Because of my specialization, there are only two groups I could choose for writing my thesis. I spoke with both and then chose one.

But now, my current professor is very hard, unfair and personally attacking me when giving feedback. This is a known issue with him amongst the other students but until my last oral test (which was not good), I never encountered problems with him.

I want to speak with the other professor and ask whether I can write my thesis in his group. I wonder what I should answer if he asks why I changed my mind. I would answer that my professor is becoming enormously rude. But I do not feel that this is the professional answer. How can I prepare for this second talk?
All you should admit is "a discrepancy, strictly on a personal level, not subject-related". That must be sufficient. You might add you made an error in your first judgment (probably due to the limited time-frame for decision) - which actually seems to be the case: so don't lay the blame on prof. A.

Even if prof. B knows everything about the conduct/character or prof. A (which is more than likely!), he or she will probably appreciate integrity - coexisting with your ability and willingness to admit a mistake and to revise your decisions in order to find a solution.

Please, don't get prof. B into a situation where he has to (or could feel obliged to) comment on the actions of prof. A. They have to get along on a professional level until retirement (while students come and go, in a manner of speaking).