How many arrows has Torin in his inventory?
Or how many other items has he in his inventory?
The inventory of an NPC has a limited size. If you fill it then you cannot give the NPC more items, be it a quest item. That's the usual explanation for problems with Torin.
If it is your case then
buy some item(s) from Thorin. This way, you will make free slots in his inventory an you will be able to give him the arrow.
As far as the bow is concerned, it is nothing to write home about. Moreover, it cannot be blessed/enchanted. Killing Torin gives you no advantage.