The long-line structure allows for predictable rhyme schemes.
God is seen as a powerful being who, when angry, does things like flood the earth. If Grendel was baring God's anger then this suggests that he is a very powerful, mighty being but since he is angry he is powerful in a negative way. The word entrap also lends to the idea that Grendel is a monster because to entrap and make way with someone or something is a very negative thing.
He means that as long is there is a wrong to be fixed by chivalry it will never die.