Depression Support Group
Depression is a real and debilitating condition that is often misunderstood by family and friends. Its meaning can range from a prolonged period of sadness to an actual mental illness with specific symptoms. Find and share experiences with others who are going through the same struggles.
In your case that girl needs a lot of professional help too, kids don't just "do" these things, they have learned it somewhere. Get help for your daughter and for yourself. Other children will have to be protected from that girl, it's so very hard because she is as you say a child herself.
The biggest quack on TV.