“…he had dwelt for a time in misery among the banished monsters, Cain’s clan, whom the Creator had outlawed and condemned as outcasts. (Beowulf 9) This line in the poem states his nature of being accused is genetic. He also suffered from being an outcast.
His internal conflict grew as he placed his hate and anger among the men of Gates and the newly built Heorot. “It harrowed him to hear the din of the loud banquet every day in the hall…” Lines 87-89
This also draws a conclusion on seeing a further look to why he brought internal conflict.
Grendel hates God and hates men who worship Him, starting from line 87, it troubled him to hear the fellowship