Grey areas are borderlines. Like when you have a villain who is sometimes good. There is nothing wrong with this at all, you just need to make sure you are doing it correctly so it does not seem out of character.
Example #1 (Evil Baddies): Your villain is a very evil, nasty, despicable scoundrel who enjoys people's pain. This is not the kind of person to give grey areas to. He cannot all of a sudden decide to be merciful or give to a children's charity or something unless there is reason behind it. Your readers will be left confused and wondering if they missed pages or something.
Example #2(Not so Evil Baddies): If you plan to have your villain turn out to be a good guy in the end, there are also things you need to avoid. He, in turn, cannot be ruthless and nasty and all of a sudden be good. You have to set up the fact that he might be a good guy through the book. He might give mercy to the hero, or if he does do something nasty, it might have been an accident or he did it to protect someone else. A good plot line with villains that turn good is to actually have two baddies. One, really evil and the other borderline so that in the end, the borderline baddie will join up with the hero and they might become really good friends. Grey villains might also come about because something happened to them to turn them bad even though, originally they were good people. My best example for this is to remember Darth Vader.
Example #3 (Grey Heroes): Sometimes your heroes have to be the bad guy to get a job done. They might have to fight dirty, be a little treacherous, or even have to torture someone. This, just like with your good baddies, all has to have reason though. As you can't have a villain be good for no reason, you can't have a hero go postal for no reason either. If he has to torture someone, it has to be for a good reason like maybe getting info out of a baddie to save a friend. He might have to fight dirty to last long enough, fighting a villain. Sometimes, your heroes are always borderline, and that is fine, as that might just be the kind of person they are. Having a borderline hero is not as jarring as a borderline villain.
Things to Avoid: Borderline villains can be all right in an adult novel, even though they are still not advisable, but you should never put borderline villains into a children's or young adult book. Don't add a borderline baddie to children's books because you're afraid to add violence. Most kids will not care as long as the villain gets his in the end. But you cannot condone things in a children's book by not giving the hero justice over a bad guy. Even if your bad guy is only accused of one crime, he still needs to be punished for it in the end. You cannot have his actions condoned. This, my friends, is a far worse example than any violence a bad guy might do in the story. Kids aren't going to love a villain if they know they can't get away with the things they do.
As for teens; the reason they are so messed up is because vampires used to be the bad guys and now they are all of a sudden good. What can you really expect?
I hope this was a help to people, if anyone has any questions, or want to know my take on any particular thing, just let me know, I'll do my best to help :)