There are (at least) five ways of looking at Beowulf's Death:

• Beowulf is indeed perfect, either as a pagan hero or as an exemplar of Christ, making his death a wondrous gift liberating his people from the old order.
  
  
• Beowulf is a secular hero and falls short of the Christian virtues demanded by the poet and as such, his death is a sad but fitting fate.
  
  
• Beowulf's eagerness for treasure and adventure is a character flaw that causes his death. 
  
  
• Beowulf dies because he is an old king, as Hrothgar was, except no young heroes will come forward to fight for him and replace him.
  
  
• Beowulf dies because we all die, and this is an elegiac poem about the giants of the past, but also about the inevitability of death.