I so totally agree that part of their attraction is how damaged they are. And about the way that JKR treated Snape in the end to be a letdown.
It's as if she had this character who ran away with his plotline, far beyond what she expected or even wanted, and then just didn't want to (or perhaps didn't know how) to deal with the consequences of what had happened to him. So, she killed him off as the "easy way out" of having to think about what he would do next, or what other characters would do to him.
OTOH, I'm really glad that she left plot holes big enough to drive a truck through. So much more for us to play with.