The Anglo-Saxon writer of this book was an early Cristian and the book expresses Christianity and what it meant to them, but "wyrd" or destiny seems to contradict their belief.
I am going to give a very different view on this. Wyrd is the trust in fate or personal destiny. According to Christian theology:
New International Version (NIV)
11 For I know the plans I have for you,” declares the LORD, “plans to prosper you and not to harm you, plans to give you hope and a future.
Therefore, all believing mankind's personal destiny is known by God. That should mean that Anglo-Saxons can believe in wryd and in Christianity because they know that they can trust in the destiny God plans for them.
I am sure that many will disagree with my interpretation, but, historically, many texts survive solely because of personal interpretation.
As other posters have said, Beowulf is a blend of pagan and Christian. This is attributable to several factors. Time is the primary one, as the poem was created and sung long before the arrival of Christianity. When it was finally written down for posterity, though, the Christian monks infused their belief system into the work. It is obvious when reading that it is a melding of two philosophies. On one hand is the belief that all is in the hands of fate (Wyrd). This makes for fierce warriors who fight fearlessly because their appointed time to die is out of their control. On the other hand is the belief that God is in control. Here it is God's timing and grace that prevails; as He wills, so it will be. Obviously these cannot both be correct, so it is disconcerting to read them side-by-side in this work.
Up until quite recently, Christianity has coexisted (and still does coexist in many places) with native religions. People have melded their own beliefs with Christianity, making their gods into saints and such. After all, our use of Christmas trees and Easter eggs come from pagan rituals.
The Christian church tolerated this for a long time at least in part because it had to. If it did not tolerate all the people who practiced these kinds of syncretic beliefs, it would have had a very hard time attracting believers.
Beowulf's original author is unknown, but you are correct in stating that an Anglo-Saxon monk first copied down this oral tale. Because of this anomaly, the poem blends many aspects of Christianity--such as compassion and forgiveness--with Anglo-Saxon and Pagan values, of which "wyrd" is one of the most notable.
Remember also that Beowulf was first written around 1000 A.D. This was a time of great change for the Anglo-Saxons, as many were converting to Europe's newest religion, Christianity. As such, the Mercian monk who penned the poem probably held fast to many of his culture's firmly-rooted beliefs, including that of "wyrd" or destiny. To the Anglo-Saxons, it would not have seemed contradictory for the tale to include both Pagan and Christian ideals.
I like to think of Beowulf as a bridge between the "old" and the "new" values of Anglo-Saxon culture. It reflects the multifarious (and sometimes contradictory) beliefs of a society in the midst of great change.
On the contrary, early Christians often blended Christian belief with existing religions in order to make their ideas more accessible. At the time Beowulf was written, life was short and often brutal. Accepting that their lives were ruled by wyrd, or destiny or "that which is becoming" helped people accept a difficult existence. It didn't preclude believing in a Christian god, as (and beware, this is guesswork on my part) it could be that a Christian god ruled their destiny and they learned to accept hardship because it was pre-ordained by fate.