The one that make the actual kill, is always the murderer; since the human mind is able to think for itself.
The others are indirect murderers at best.

The exception might be with a mentally ill person: if he has little or no free will, then he might be a tool, just like a gun is.

In case nr.9, B is the murderer, although he should not be punished for his action if killing A was the only way, as he saw it, that he could survive.