Why is it called the book 'of Night with Moon?' Because a moonlit night is when the contrast between light and darkness is clearest and most iconic. It's a fitting name, given that wizards spend their whole lives pushing back the darkness.
In the Young Wizards/The Book of Night with Moon universe, the Lone Power seems to like killing off the protagonists' family members to try and push them over the Despair Event Horizon or make them desperate enough to cut a Deal with the Devil. The protagonists cannot possibly be the only ones this has happened to. For that matter, It has been known to extinguish stars to hinder the protagonists, killing off entire civilizations in the process. Once again, It's probably not giving the protagonists special treatment here.
It doesn't help in the least that it is made explicit that there is an afterlife, and it's a much nicer place. Hello Unfortunate Implications on the morality of keeping somebody alive in that universe...
Unfortunate Implications on the morality of keeping somebody alive in that universe...