Also, why did they get so much nicer in modern tales?

So, the most common answer I’ve seen given to why fairies suddenly became nice is that it’s to do with our society’s changing attitude to children. From about the sixteenth or seventeenth century on, ideas about childhood began to change until by the Victorian period childhood is seen as a time for happiness and something to be cherished (at least if you weren’t poor and forced to work as a chimneysweep or down a mine). And with that change in attitude children’s stories became more about humor and carefree flights of the imagination and less about people dying all the time.