"each street pulses." and "beneath a skin" suggest the street has a pulse beneath a skin, which we know is not something a street has, but humans do. Personification is when a non-human thing is describes to have the characteristics or aspects that humans have. Which is exactly what the poem is doing, leaving us with the conclusion that the literary device used is Personification.
It signifies that Grendel is cursed by God and is inclined to commit crimes like this because of his family. Grendel in general is at odds with Christian civilization.
