Middle Grade vs Young Adult

Okay, I was going to just google this, but I've found that this site is the most helpful writing resource out there for me, so I thought I'd give this a try.

I am currently working on my second novel (technically it's my forth, but if it's the start of a second series) and I think it's a really good idea. I'm a person with a lot on my mind, so it's going to at least be like four or five books at the least.

I've started writing it as a middle grade novel, but I'm not sure if I should keep it that way and just make it young adult. The themes are simplistic: good vs evil/light vs dark kind of thing. I guess you can call it a fantasy, but not it's not creatively another world type deal.

The characters, what happens to some of them, the relationships, and some of the bad guys are pretty dark. The good guys are corrupted, tempted, fall into darkness, and so on. That's why I'm thinking that maybe it's not middle grade appropriate.

I did read that it can depend on your main character, but the main characters of the story ranges from seven to fifteen.

I guess if I'd have to compare it to anything, I would compare it to Avatar: The Last Air Bender.

Feedback, please!

Of course I'm not planning to write about sex, but the relationships are one thing. The kids grow up throughout the books and even before that there are relationships. Nothing evolves past kissing, but there is a lot of relationship stuff and some of it can get kind of twisted.

People are going to die, there are wars, there are quests for revenge, there's Japanese type mafia people, and there is like a concentration camp for some types of people. It's nothing compared to the Holocaust or near that brutal, but people are picked out for being different, taken from their homes, and kept as prisoners in camps where they are treated as freaks.