Grendel can be considered to be a modern-day monster primarily because there are so many human characteristics in him, and in the old days that wasn't the way in which monsters were portrayed. Here we can see that Grendel feels hatred towards the soldiers, that he is also afraid when he sees Beowulf, he isn't just a mindless monster lusting for blood, there is a reason behind him doing all the killings.
After reading this you would have got your answer :)
Hope this helps :)
