Why was King Hrothgar in despair after the several murders of his kinsmen?

I believe you are referring to the attack by Grendel's mother in which Hrothgar's best friend and advisor, Aeschere, is killed. Hrothgar is mourning for the mny who have died, and the fact that Grendel's death hasn't ended the slaughter. Beowulf tells Hrothgar not to give into despair, as "it is better to avenge our friends than to mourn overmuch".