Complex sentence: The last sighting of Beck was by one of the survivors, who saw him slumped under a tree with his head to one side, some time before the end of the battle; they couldn't be sure that he wasn't already dead at that point.
Split into simpler sentences: The last sighting of Beck was by one of the survivors, who saw him slumped under a tree with his head to one side, some time before the end of the battle. They couldn't be sure that he wasn't already dead at that point.