When we dream, all the different parts of our 'inner selves' interact, and 'come alive', as though they were different people(known and unknown alike).

Friends - Our dreams are full of symbolism with their messengers being all different types of people. It is very common to dream about our friends. These are individuals that are emotionally valuable to us, and we learn about ourselves through them.
This dream is more than likely attempting to bring up uncomfortable feelings that you have about yourself or others. (Remember "uncomfortable" does not necessarily mean bad.)
Most likely there is a attribute of your friend that you may see within yourself.

Hitman (assassin) - Those aspects of one's personality that tend to "kill" off potentialities. Subverting realizations of one's goals by one's own actions or by outside forces. Suggests you feel that you or your friend may be sabotaging yourselves in areas of your lives...( jobs, relationships, etc)...though your actions, or the lack thereof.

Alternatively, even though you wouldn't harm your friend for the world....there are times when you might be subconsciously angry with her, but not have any clue of it within your waking life.

I would liken it with something akin to sibling rivalry. We love our sister, and brothers....and would lay down our lives for them. But, there are times when we have a dream about doing something awful to them.
I wouldn't worry about it too much unless it becomes a recurring dream. Then it wouldn't be a trivial matter, but one that is troubling you deeply.
I hope this helps un-spook you!
Hugs & Happiness,
J