why did beowulf decide to free the danes rom grendel

It wasn't a decision as much as an obligation. His father owed Hrothgar a debt of loyalty, and Beowulf "inherited" that debt. All ancient societies took vows, debts, and ancestry very seriously indeed!

Also, fame and glory in those days were everything. Anything a man could do to achieve glory, he did. It was part of the heroic code of the day.