There are Biblical examples of forgiveness for murder including the story of David and Moses. The story of Moses can be found in Exodus 2:11-15. Legally I suppose it would actually be considered "manslaughter" as there was no premeditation in the killing of the Egyptian. In the case of David, found in 2 Samuel 11, there is clearly premeditation. Yet in both cases, God is able to forgive these men and still use them for His purposes. Now mind you, their deeds are by far not without consequences. I the case of Moses, he spent the next few decades of his life far from his home as a shepherd (quite a change from his early life living in Pharoah's palace). David's consequences were much more severe as he lost the child born to Bathsheba and the sword would never leave his house (2 Samuel 12).

What we can pull from this is that our sin is never without consequence but no sin is too great that God can't forgive the repentant heart.