When unferth says "the sins of the father" he is missing the second half of the quote which is "shall be visited upon the sons". how does this quote apply to the movie? would it have applied to the anglo-saxon culture?

The quote applies to Beowulf's movie with a cause-effect relation about his actions and wrongdoings, a mere punishment by not acting "well", a sin. It would have applied to the Anglo-Saxon culture of the time Beowulf was written because it is believed it was written between 700-1100 AD, while Christianity was introduced to England as an Evangelical attempt since 597 AD. So it describes the mix of paganism and Christianity lived in the era.

Explanation:

The phrase "the sins of the father shall be visited upon the sons" is a biblical quotation that says that all the wrongdoings of the father according to the word of god are going to be passed to the sons and daughters of the sinner.

It shows the mix of paganism and Christianity that was lived in England at the time. Also, because it is well known that clerics and the privileged were the ones with access to education. Suggesting that Beowulf was written of developed by a person who knew enough written language that could have learned from the bible.