Death and war go hand in hand, and For Whom the Bell Tolls is about war. So almost every major character in the book is forced to come to terms with their own death, and the deaths of their loved ones. They display a variety of attitudes towards death, from paralyzing fear and despair, to resignation, to relative unconcern. One thing that all experience, however, is the way death forces a re-evaluation of one's priorities. The protagonist's attitude towards death changes as the value he gives to his own life changes, and the book explores the relation between the two.
At the start of the book, not only is Robert Jordan not afraid of death, he actively wants to die.
Pablo's fear of death is actually a symptom of a more general despair caused by his guilt over the people he's killed.