Before jumping the gun on taking her to a therapist, you need to evaluate her emotional ability first.
Everyone has bad dreams, especially young children. It's part of growing up.
Now, while she's having dreams about killing people she knows, that's not necessarily something you should start paying a counselor to solve. A lot of these issues can be solved by talking to the kid themselves. Six months is a relatively short period of time for recurring nightmares, which are usually caused by some aspect of the persons current life.
It's important to make sure they understand the difference between reality and their dreams, which I'm assuming she does since she is 11.
It's also important to not make the child feel as if something is wrong with them. Taking them to see someone "special" can do more harm than good. As someone who had similar experiences, being forced to take medication and see a doctor every week can be very depressing and debilitating. Children at school-age are very susceptible to peer-pressures and teasing, which can cause self-image damage that only further deepens their issues, and feeling "wrong" amplifies this.
Since her behavior hasn't changed, there doesn't seem to be an issue.
In my personal opinion, the only thing you should do is reaffirm your child that it's alright, it's "just a phase," or some other form of comfort. A child's trust and a parent's word can go a long way. However, keep an eye on their behavior and if anything really out-of-character happens, then reconsider.
As a side note, I would like to mention that many children are heavily influenced by their parents, even going so far as to have mini placebo-like effects. Perhaps you could consider some form of "solution" that you had as a kid that helped the exact same problem, such as something silly like sleeping with a teddy bear. It is lying, however, the fact that the child's parent is saying it will work can be a powerful 'fix' for the child who believes it.
I remember my own grandfather told me that if you take the stem out of an apple it's perfect to eat, and so naturally, being 8, I did that for every apple I ever ate.