Grendal had just barged into a fortress where all these men were sleeping one night and started ripping their limbs off. The first line shows the panic and urgency of the men as they were abruptly woken by this giant thing out to kill them. The second line shows that Grendal killed thirty of these men who had been recently unaware of his existence.
The first two lines give the reader a feeling of dread from Grendal before they even know for sure who he is. It is slightly obscure in how it describes it, and makes the reader tremble as they realize these men were jostled awake only to be killed minutes later, while their comrades watched in terror. At this point the reader sees Grendal as a terror of the night, swooping in to devour and quickly leave.
The third line adds a bit more to his character. It shows that what he did wasn't a primal instinct. He wasn't just some animal who had no choice but to obey instinct and attack. He did this with a purpose. He enjoyed murder. He was joyous, "leaping and laughing," as he left. He had a lair that he had most likely built with his own two hands to return to. He had a brain, and while it may not have been the brightest one around, he knew how to use it.
The final line almost summarizes the first three, in a way. It shows that after having killed thirty men, he felt invigorated as he headed home. All of these aspects combine to make the reader feel as though Grendal is a monster, and not just a monster with animal-like instinct, but a monster who had human tenancies without morals.
