How can we understand Hrothgar's sermon as a guide to proper kingship?

Chapters 24 and 25 (Lines 1686-1784) are often referred to as "Hrothgar's Sermon," which seems to be precipitated by Beowulf having handed Hrothgar the hilt of the sword Beowulf used to kill Grendel's mother. Hrothgar begins with comments on the runes, which describe the destruction of the giants by God's flood, but then immediately launches into a lengthy discussion of kingship. As the poet did earlier, Hrothgar uses Heremod as the model of a bad king, a man so obsessed with power that he mistreats his own people, fails to distribute wealth to those who serve him, and is eventually cast out of his own kingdom. Hrothgar's purpose in this discourse is to show Beowulf how he is to "become a comfort/for a long time to your people,/a help to men" (ll. 1706-8).
In one of the poem's lengthiest discussions of man's fate, Hrothgar warns Beowulf to resist "the wickedness" of unbridled power because "the glory of your strength/is only for a little while . . . sickness or weapons will steal your might" (l. 1763). In other words, life on this earth is fleeting and subject to many kinds of disaster, a theme that runs throughout the poem. Hrothgar ends this discussion by pointing out that even though he ruled wisely and well, his own kingdom suffered at the hands of Grendel and his mother, and he thanks God—whom he calls "the Governor"—for his salvation. This last piece is undoubtedly the monk-composer reminding his audience that this is a Christian poem, but the sentiment is consistent with other such statements in the poem.
The importance of the "sermon" is that it sets out for the second time in the poem the requirements of good kingship, using Heremod as the primary negative example of kingship gone awry. More important, however, is that Hrothgar, a man whom Beowulf respects, speaks directly to Beowulf about the dangers of pride and lust for power, balanced against the elements of good kingship and, ultimately, the unavoidable end that all men must face.