In response of two or three sentences explain how these lines contribute to grendels characterization as a monster but little from malice and murder they mastered him fully

I believe the lines are:

"Not longer he tarried,
But one night after continued his slaughter
Shameless and shocking, shrinking but little
From malice and murder; they mastered him fully."

So, Grendel is not an ordinary human being. In fact, he is not human after all, but a murderous, villainous monster that defies both men's and God's laws. His slaughter is not a monstrous element - after all, people did it too, especially warriors. What illustrates him as a monster are the words "shameless and shocking, shrinking but little from malice and murder". The epic world of "Beowulf" couldn't possibly be shocked by an "ordinary" murder or any act of violence. What makes Grendel's murders shameless shocking is the fact that he doesn't commit them because it needs to be done. He commits them simply because he can, and enjoys greatly in it, even devouring the corpses afterwards.

In my opinión this is the first person

This would be third person because they are using the correct pronouns that belongs to third person like he, the name, etc