Your question was, "How could a person die if they had a pacemaker?"

The pacemaker only replaces the electrical system in the heart. If a person has a heart attack, for example, the oxygen is cut off from the muscle which will cause the heart to stop because of "mechanical" reasons. Heart muscle needs huge amounts of oxygen in order to do its job and when that oxygen cannot get to the muscle, it dies. The pacemaker can only 'pace' good heart muscle.

Hope this answers your question.
Mr. D