Super interesting! The cave was where Beowulf met with the greatest success in killing Grendel's mother. When he fought Grendel in the hall, he did manage to inflict a fatal wound, but did not finish the job there - instead he got Grendel's arm as a trophy to nail up in the hall. But it was seeing her son, limping home to die, that set Grendel's mother off, causing her to seek revenge against the humans. His fight in the hall just led to more bloodshed, and not just of any old retainer - She came and carried off Aeschere, Hrothgar's great counsellor.
Beowulf's fight against Mommy Dearest actually took place in her cave, and his coming out of it and out of the water after killing and decapitating her sounds very much like a birth:
"Immediately the counsellors keeping a lookout
with Hrothgar, watching the lake water
saw a heave-up and surge of waves
and blood in the backwash." (1591-1594)
Of course, Hrothgar and his men interpret this to mean that Beowulf has been defeated, so they leave, broken-hearted, and it is Beowulf's troops who later see him come out of the water with the bloody head of Grendel's mother.
Finally, Beowulf's end comes when he faces the dragon, not in the dragon's cave but out in the open. Maybe Beowulf could have ruled another 50 years if he had just gone in and taken the dragon inside a safe, nourishing cave! :)