Modes found in Beowulf

The poem Beowulf contains two main forms of modes, Heroic and Christian, which occur frequently throughout the whole literary piece. Heroic mode tends to focus on what would be considered “heroic” traits and acts such as revenge, war, and bravery. The poem definitely switches in to a Heroic mode whenever Beowulf is being described or is present in the story. In fact, a great example of this would be lines 190-210 from the section “The Hero Comes To Heorot” where Beowulf bravely prepares to save a once powerful town from the demon Grendel who has plagued and rampaged the town for twelve long years.  

As soon as Grendel enters the story though, the mode typically changes to more of a Christian feel because typically a biblical reference is made whenever he is described such as being described as a "fiend out of hell". In lines 90-115 in the section labeled "Herot is Attacked" it even brings Cain and Abel in to the story by stating that Grendel was actually present at the time that it happened and goes in to the goodness of God and how the world came to be. 

What I've really enjoyed about Beowulf so far is just how seamlessly the two modes are used throughout the entire poem. Sometimes I don't even realize the fact the mode has even changed when I am reading it and I really look forward to finishing the rest of it. 

Questions: 

1. Why did Beowulf wait so long until he finally came to help the town being rampaged by Grendel? 

2. There is a section in the book that says "Out of the cures of his exile there sprang ogres and elves and evil phantoms and the giants too who strove with God time and again until He gave them their reward". What the heck does the even mean? What reward? 

3. After all this time, what made Grendel suddenly decide to attack?