When Grendel fights Beowulf the reader learns that the monster's "time has come", and he will soon die. What does this idea say about the Danish belief in fate?

I can't write your narrative for you but consider fate for what it really is. Nothing can change the outcome so one can either accept it or rage a futile battle against the inevitable. Grendel, somewhere in that monster heart of his, knew Beowulf would be his undoing; life also sucks if you are missing an arm. In any case, consider the weight lifted from you when your fate is sealed and the ever present temptation of trying to change it.