Whether it be in sports, in the classroom, or in relationships, he wasn't happy unless he beat someone else at whatever he was doing. Being in competition to Mark meant that he was going to do whatever he could to ensure he won and that his competitor lost. Those on his team loved his competitive spirit and encouraged it; those not on his team feared him. His behavior was validated through the number of trophies and medals he received while growing up.
As an adult his competitive spirit didn't wane. He became a feared negotiator in his company's purchasing organization and became known as a pompous ass who would stop at nothing to ensure that his suppliers were giving up as much as possible so his company could get a better deal. Suppliers hated to deal with him, but his company was too big to ignore so they put up with him.