I read your second post & after reading that, and re-reading the poem it sounds a lot like it's talking about a vampire. Vampires are extremely popular with teens & pre-teens right now. Everything fits in if you look at is as describing a vampire & vampires are supposed to live forever. Usually in pop culture they are connected with each other & win their true love. I can't help but wonder if it isn't just based on our youth culture's idea of vampires being romantic & nearly invincible. It's an alluring idea when you're young.
Definitely messy divorces are hard on kids, though. I would tend to think that the poem could be a positive coping technique, but I don't know her. Personally, I had a nightmarish childhood & going goth was a way to rebel against my parents & connect with people who understood that life isn't all rainbows & puppy dogs. It made my mom SOOO mad, but I actually became much less depressed b/c the people I hung out with were more resilient than most. There are different groups & often they are all lumped together by "outsiders" -- goth, industrial, emo, loners. If she is connected with a caring group of friends -- regardless of how they dress or what kind of music they listen to or what kind of poems they write -- that will insulate her from many things. Nothing is perfect. I lost 2 classmates -- 1 a good friend -- to suicide in HS and both of them were popular kids with good grades & Gap clothing. The kinds of writings they wrote were despondent -- nothing matters, nobody cares, I am empty inside. As a teacher, I went through hours and hours and hours of training on recognizing signs of school shooters & suiciders. The primary risk factor is not having a group of friends, followed by not having teachers/parents/coaches, adults, who care about them, want the best for them & will listen to them. There are lots of other risk factors, but good relationships can overcome a multitude of negatives (divorce, death of a parent/sibling, homelessness, serious illness, etc.).
I think your granddaughter is lucky to have you. I know my gma was a great help in my life. She's suffered from depression too so she didn't just tell me to get over it; plus, she didn't tell me to keep the beatings/burnings/whippings a secret. She tried to get custody of me & even though she failed at least she tried & that meant a lot. She shared ideas about how to barricade myself in my room, suggested school activities so that I could be out of the house as much as possible, send me cards & little gifts (prayer cards, guardian angel pins ... just little stuff that I know she put some thought into). She's a major reason why I made it through high school -- that and my friends. I wasn't allowed to go for counseling & had to sneak out to go to church. So I really had to rely on people who weren't experts but were nonetheless really great at listening, encouraging & loving me no matter what. I am so grateful for my gma & I'm sure your granddaughter is really grateful for you. :) Letting her know that you're sorry for what she's going through with her parents right now & that she can always call you to talk & you will always listen and try to help her if she wants help will go a long way towards helping her get through the divorce (her parents may not love you for it, but it is incredibly healing).