What is one negative affect of Billy’s rescuing little Ann?

I don't see a negative side to saving Little Ann. Billy's ingenuity as a hunter is demonstrated as he smartly uses the lantern to save Little Ann. However, he once again receives help immediately after praying to God. So long as Billy prays from the heart, as his mother says, some mystical force seems to aid him.

This is the first occasion of true life-and-death matters. There will be many more throughout the book, and Billy, Little Ann, and Old Dan all take turns as the victim and the savior.