I disagree, there's plenty of "grey" characters in LOTR and the Tolkien universe. Boromir is the obvious example. He's generally a good dude, and even saves the fellowship's ass on a couple occasions (on caradhras) but is plagued by doubt and the pull of the ring. But even Aragorn messes up, and avoids making decisions after leaving Lorien which leads to the events on Amon Hen. I see what you're saying, but I don't think the moral dichotomy is as stark as you're making it out to be.

If you want a dark or 'gritty' take on middle-earth, I highly recommend the Children of Hurin. There's murder, incest, betrayal, suicide. I just started reading it a couple of days ago and finished it in a 24 hour span. Really enjoyed it.