one month ago, my friend told me she had depression because she felt left out. I made an extra effort to include her, but she recently sent me a letter saying that she wanted to kill herself because of me. What can I do?

You talk to your mom or dad or her mom or dad about this situation. Just know that it isn't anything that you're doing or not doing. It's a bid for attention. By saying that she's going to kill herself since she feels left out, it made you consciously include her.

Now she's saying that she's going to kill herself because of you. What does she want you to do? Kill yourself as well?

Tell her that you've tried to help her by including her but now she says she's worse off, so it's better if she were to talk to her parents and get some help.

Some one who really wants to commit suicide will not tell anyone about it. So to me this is a good thing cuz she is seeking help BEFORE it gets worse. My opinion is that you gave her what she needed to feel good the first time so now she is seeking you out again. Danae is right, you need to get her parents involved.

Whatever else you do, be aware that regardless of what she says, you are in no way responsible for what she may do if she decides to harm herself in any way.