-Grendel may have led a painful life for several of reasons. Mainly because he was a descendant of Cain. Meaning he was born evil no matter what. Also, people compared him to a beast which made him lack confidence in himself. Last but not least would be because clearly Beowulf came to kill him and that is alot to endure. OR you can word it this way
-One reason Grendel led a painful life is that he was a descendant of Cain. From the beginning he was born out of pure evil. Another reason Grendel may have led a painful life because he was compared to a beast. The motives of this beast was to stop any festivities in Heorot Hall; which caused him to be a primary target