it's relatively standard. The first draft was even friendlier than its current incarnation. Adding those dark scenes during editing, and the small bit of violence has been incredibly enjoyable.
Why is it so fun to write horrible/terrifying things into a book? I think the main reason (for me) is catharsis. There is something so cleansing about putting a character (even one you love) through the ringer. I find it easier to dump the stresses in my own life onto their fictional shoulders in the form of monsters in the darkness and painful injuries than it is to carry it around myself. They cannot suffer like a real person of flesh and blood, and for those moments as I write, I can forget my own suffering (that sounds a little over-dramatic, but hey, it works).
Another reason is the need to see your characters succeed. In real life, you would never want to endure watching people die around you, fight in a horrible battle, or crawl through a slimy hole in the ground filled with terrifying creatures, just to feel like you succeeded at something. We like our experiences a little bit smaller in scale, more manageable, like getting an "A" on a test. But in fiction, the characters can endure such things, and their success is almost as fulfilling to the writer as if they had done it themselves. Seeing your character grow, knowing that they are becoming more and more real is a point of great pride and pleasure to a writer. The same can apply to the reader.
So the next time you find yourself wondering why so many things are happening to such a nice character in the book you are reading, and why this author is so sadistic, stop and think about why they are doing it. You might find that you wouldn't want it any other way.