Is the student sinful for retorting to the teaching assistant who insulted and threatened him with failure in front of the other students, and what is the solution for dealing with this situation?

You did not err in demanding your right from the one who insulted you, but you were mistaken by abandoning the exam. You should have completed it and spoken to the teaching assistant gently, or escalated the matter to a higher authority to seek justice. We advise you to claim your right in a good manner, without foolish acts that would prevent you from doing so. The person should also be admonished and advised to improve their manners.