"Ivan Ilych's life had been most simple and most ordinary and therefore most terrible"

This would mean that Ivan was just a typical average Joe.  He studied hard to attend law school.  He only married just because he had to fit in with the norm of the society.  He had a mid-life crisis with his wife that caused him to stay further away from her with the excused of working.  Because he spend more time at work then at home, he was able to get promotion and eventually became a judge.  All of these characteristics were simple but terrible in some ways.  He did things base on what he believed others wanted him to do, not from what he wanted to do.  Thus, he lived a misery life that in the end he had to question whether he had done any wrongdoings.  If he had accomplished all the things that he wanted to, then he would not have to be hesitated to face death.  In the end, his life was most terrible because he lived for others and not for himself.