I would also suggest therapy and or counseling. Also, be very involved with her life. Show and take her places that allow her to express herself in different ways. Teenagers often are a little like the babies we once knew. Once they figure out how to shock you or push your buttons, they will push them like a 3 year old in an elevator. But since your grandaughter comes from a drug background there may be some kind of an abuse issue that she is acting out in defiance of God, who did not protect her. Of course, these are just thoughts of mine.
I have 3 children, my most recent is only 2 but my oldest is 22 and my daughter is 20. When my son and daughter got around the age of 14 or so they were also fascinated by the dark side of things. My son would draw some of the most gruesome pictures. My sister, a very staunch JW at the time, use to say all kinds of awful things about my children, but truth is, his pictures were very, very good. He obviously had talent. I tried to get him to draw pictures that were nicer, he tried, but it just wasnt his style. So to appeal to me, he drew some very frightening armageddon type pictures that were just as gory as his gothic dragon like ones. I backed off, I realized that he was trying to express himself. He wasnt doing drugs and I guess as a parent we can always overreact when out babies start expressing themselves with different views than our own.
I was very involved with the truth and I think this was why they were rebelling. My daughter even took things a step further and did other things that just blew me away. She really knew how to scare me. She would cut herself, sleep with different people, throw fits of rage, take pills. I would just have to keep up with her and in her face all the time. We had her in therapy, I kept myself tough in front of her and would cry in private. I had to be stronger. I just kept myself involved with both their lives as I would compete with their teenage friends. I blame some of it on the way I taught my children through the organization. Everything was about surviving the armegeddon and that God would destroy those who did certain things. I should have focused on the love. I blame myself too. I started studying when my daughter was 2 and hence the religion the way I saw it was ingrained in her. I took a lot of her childhood away with too many meetings. No balance. No fun.
Now they are both young adults, the phase is over, and they are very well rounded young people. Not perfect, not JWs, just decent, loving young adults. All the crazy things my kids did when they were teenagers are just the past. Neither does drugs and each has respect for family and friends. This of course was my situation, I just hope you are able to draw some insight. Your situation may be completely different, you know best.
Try not to be so shocked at her behavior. When we show our fear, kids can read us like an open book, heck they have been reading adult behavior since they were babies.