Conflict, skillfully handled, is one of the keys to a great relationship. Conflict out of control is an excuse for physical, verbal, or psychological abuse. I coach people to take turns being angry, and when both partners are angry, take a time out. Criticism is usually a sign of that the criticizing partner has some personal development work to do. I coach that it is OK to complain, but not OK to criticize.
Contempt is criticism escalated to outright mental abuse.
Fear is natural. Defensiveness naturally accompanies fear. Skillful partnering requires practicing techniques that allow me to drop the defensiveness despite my fear. When I deny my responsibilty for my part of the issue, I wind up blaming my partner and trying to change him or her. Remembering mainly the negative experiences in a partnership is a predictor for future breakdown.