Jack is afraid at the beginning.  He isn't quite sure about whether he can kill a pig and he is nervous at some points about the beast.  But he starts to realize that fear is his best tool to bring the boys to him, whether it is a fear of the beast or fear of going hungry, etc.  So this fear becomes far more effective than Ralph's appeals to reason and order.

As the boys start to get more and more desperate, the fear of all these things grows on them, particularly as they start to feel that they might not get rescued at all.  By this point Jack has also lost much of his former fear as he is confident in his ability to hunt and his ability to make the boys follow him.