After Grendel is defeated what follows that brings further sorrow to King Hrothgar?

King Hrothgar becomes sad because he foresees the trappings of Beowulf's hubris. Beowulf chose to fight Grendel barehanded to showcase his great strength and bravery. King Hrothgar knows that a truly good king would never take such an unnecessary risk simply for pride. See the link entitled King Hrothgar in Beowulf-King Hrothgars speech to Beowulf in Hall of Heorot in the related links section for an analysis of Hrothgar's warning about hubris.